After a Senate estimates hearing on Wednesday raised questions about the extent of the Treasury's involvement, Prime Minister Scott Morrison was forced to assure the nation of the public service's input into modelling for the Coalition's net zero policies.

In responding to questions, Morrison name-dropped Industry Department head David Fredericks, while Deputy Prime Minister Barnaby Joyce endorsed the department's expertise in advising on future demand for Australian coal and gas: "The modelling seems pretty good ... departmental officials didn't just walk off the street."

The Commonwealth public service has played a major part in the nation's attempts to address climate change since well before the issue began felling prime ministers.

It's a political and policy matter that at times has brought turbulence to the public service. Senior bureaucrats have been sacked for their closeness to climate policy, and public servants have had to adapt their advice as changing governments swung from one stance to another.

Despite the fallout, some of the nation's former senior public servants - including those who were closely involved in climate policy - still encourage people to join the public service if they want to work on politically contentious policy areas.

They offer these tips for public servants: Don't be cowed, be ready to develop alternatives, use the right language and don't lose sight of your "true north".

The public service has left its mark at the most crucial junctures in Australia's climate debate - including at its beginnings in the 1990s, when it began developing carbon pricing mechanisms for governments. Its experience since has been one marked by policies carefully devised and, later, swiftly demolished by politics.

Public servants played important roles, alongside Howard-era environment minister Robert Hill, in the Kyoto negotiations. Australia was among the first countries to sign up to the Kyoto Protocol, earning Hill a standing ovation from cabinet on his return.

The environment department secretary at the time, Roger Beale, recalled getting a nice letter from Prime Minister John Howard too for the occasion.

With the threat of climate change finally gaining traction, the Environment Department looked for the most appropriate mechanisms to deliver on the government's 1997 Kyoto Protocol promise.

Beale, who served between 1996 and 2004, tells The Canberra Times he and his senior colleagues wanted to see a price on carbon in some form as well as regulatory changes. In the late 1990s, there was a great deal of hope and promise. Howard tasked Hill and his department to begin developing an emissions trading scheme.

The public debate shifted when conservative politicians brought the cost and immediacy of climate action into question and the government became more hesitant to act on the science-based policy advice it was receiving.

Beale says he didn't feel like his department's advice was falling on deaf ears but it certainly wasn't being used.

"The ears were listening but they weren't agreeing with me," he says.

"That is, of course, the privilege of ministers and prime ministers. Ours is to propose, theirs is to dispose."

It wasn't as simple as advice offered and advice ignored.

Priorities shifted for Australia as attention turned to national security following the September 11 attacks. There were also fears that a strong commitment to reducing emissions would jeopardise a $25 billion gas deal with Chinese authorities off the Western Australian coast.

In the end, a move to ratify the Kyoto Protocol was knocked back and meaningful action to deliver on promises to limit the country's growing emissions put on hold.

During the interludes between serious government consideration of carbon pricing, the public service kept thinking about the issues. It wasn't until 2006, when he was faced with community pressure and growing momentum from the states on climate change action, that Howard moved seriously on a market mechanism to reduce emissions.

He asked his department's secretary, Peter Shergold, to create a task group on emissions trading. It was a pivotal moment in the public service's involvement in climate change policy.

The group, chaired by Shergold, appointed senior public servant Martin Parkinson to head its secretariat, and included department secretaries and industry representatives. Initially criticised by some as an exercise in procrastination, the review went on to recommend the government introduce an emissions trading scheme no later than 2012.

Treasury did the modelling. Parkinson also drew on the thinking that had already happened inside the public service on an emissions trading scheme. Both major political parties went to the 2007 election promising an ETS.

The rest is well-known political history: The scheme failed to pass Parliament under the Labor government. When the Gillard government later embarked on a carbon price, Treasury again provided the modelling and contributed to developing key parts of the policy, including delivery of assistance to households.

The pricing mechanism, which drew on and modified the previous emissions trading scheme policy, reflected nearly two decades of intellectual input from the public service.

Another pivotal moment led to its demolition. The Coalition returned to government in 2013 and abolished the carbon price the next year.

Tony Abbott's decision to sack Parkinson as Treasury secretary reverberated heavily inside the public service. Abbott, against the wishes of then treasurer Joe Hockey, and the advice of Howard and former treasurer Peter Costello, dismissed Parkinson for following the directions of previous governments in advising on and implementing climate change policy.

"It was all about paying debts that Tony Abbott had incurred, paying debts around climate policy, and one of the debts was to execute me as the analytic leader of the development of that policy," Parkinson says.

Parkinson saw the reaction to his sacking within the public service.

"People were aghast, and were saying to me 'this really raises a question about whether or not you should get involved in high profile policy issues - am I better off keeping my head down? Not trying to really push the policy agenda that a government might have, if that policy agenda is contentious with the other side of politics?'," he says.

"Others were saying, 'Well, if this is the way you get treated for doing your job, and helping a government implement what had been bipartisan policy, and helping a government that had been legally elected and had the power to make these decisions, should I stay in the public service?'"

After he was sacked, Parkinson was asked to stay on as Treasury secretary for another 15 months to help prepare for the G20 summit among other major priorities.

To those asking whether they should stay in the public service, Parkinson's response was and continues to be this: Absolutely they should.

"There's nothing like a head on a pike outside the castle gate to send a message to try and cow people. Don't be cowed," he says.

"You've just got to keep sticking to the sense of what is the right sorts of policy directions. It's up to governments whether or not they choose to follow your advice, but you should not change your advice.

"Now, if what they do is they say, 'I've heard you on X, and I'm not going to do it, give me alternative suggestions', well, then as a public servant, that's where you use your smarts to try and develop alternative ways of skinning that cat that actually satisfies the needs of the government of the day.

"But it doesn't mean you lose sight of your true north. That's where you do that policy work, and you put it in the bottom drawer. And when the context changes, you pull it out, you dust it off and say, 'here's an idea'."

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Shergold also offers this advice for public servants advising on politically sensitive issues: Find the language that is most likely to be effective.

If he had designed the proposal for an emissions trading scheme for the Howard government to consider, and couched it in language later used by Kevin Rudd - "the greatest moral challenge" - it may not have been accepted.

"It was entirely appropriate for me to couch the argument for an introduction of an emissions trading scheme on the basis that this was, in effect, a cost effective economic policy based on reducing risk for future generations," Shergold says.

It's not about changing the arguments.

"It's trying to think what are going to be most effective in terms of the directions that you can see being set by the government," he says.

Shergold also recommends having advice to government that doesn't only seem the views of well-trained, senior public administrators. It was important in his experience to collaborate with private and not-for-profit sectors, and make them partners in policy proposals to government.

More recently, the public service has carefully monitored changing business attitudes towards carbon reduction.

Public sector bodies such as the Reserve Bank of Australia, the Australian Securities and Investments Commission, the Australian Prudential Regulatory Authority, and PM&C have all paid attention to discussions on climate change. Treasury is engaged with the issue, although department secretary Steven Kennedy last week said it had not modelled the effects of climate change and emissions reductions on the nation's economy for years.

Department of the Prime Minister and Cabinet secretary between 2008 and 2011, Terry Moran says the Commonwealth has to resume leadership on climate change policy. Whichever party wins the coming federal election will have to do so in the next term of government.

When it does, public servants should be ready, Moran says.

"At some stage, the Commonwealth is going to have to get serious, and it will need public servants who are informed about developments in what is in effect a very complex technical area," he says.

"If I were in PM&C now and given the imminence of a federal election, I would be asking for the red and blue book briefs to be prepared with a stock take of where things are at in various Commonwealth authorities, and in state and territory governments, as well as in the discussions overseas about what is to be done, most particularly the Glasgow summit. There is a lot to report on.

"So an incoming minister and prime minister for that matter ought to be presented with a brief which says, 'here's the picture, and here are the matters that the Commonwealth government could give priority to, as it seeks to provide leadership in this area and help ease the coming inevitable economic transition'."

The abandonment of carbon pricing in 2014 changed the game for federal public servants advising and working on emissions reduction policies. Focus has shifted from a large, central plank of policy to reduce emissions - the carbon price - to a suite of policies that aims to reduce emissions piece by piece.

Still, the path to net zero emissions is an uncertain one, as shown by documents last week outlining the Coalition's net zero policies.

Morrison assures Parliament that public servants are on the case. He might be betting on it.

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