The lobbyists represent interests big and small, left and right and everything in between. But they all have one mission: to influence politics. Hundreds of millions of dollars are spent each year by their clients in a bid to be heard in the corridors and offices of Parliament House.

What is this lobbying class? Who is in it? What do they actually do? And what are the potential pitfalls for the rest of us, whose influence on politics usually extends to voting every three years?

Lobbying is "communication in an effort to influence government decision-making", according to the federal government's lobbyist register.

Lobbyists are employed by clients - from ASX100 companies to small not-for-profits - to talk to the government on their behalf. This might mean seeking face-to-face meetings with politicians, calling their staff or speaking to journalists and senior public servants.

While the meetings often take place in the office suites of Parliament House or the restaurants of Civic and Kingston foreshore during sitting weeks, lobbyists are also easily spotted among the tables at Aussies Cafe at Parliament.

Lobbyists will be engaged for a whole range of reasons. A group or business may just want to have a relationship with and "be known" to the government. They may have a particular problem with a bill or a policy that affects their business or interests. Or they may want their point of view heard as a policy is developed.

One lobbyist described their job as "translation": they understand how politics, Parliament and Canberra work, they know who to speak to and can use those skills to try and solve a problem for their client. This specialist knowledge includes the complexity of federal-state relations (who is responsible for what), the role of public servants, what power ministerial staffers have, how legislation works and where different MPs are likely to line up on different issues.

Two recent studies have found high - albeit differing - numbers of people go from government to lobbying work.

Australian National University professor Darren Halpin has found that 56 per cent of registered lobbyists have previously worked in government.

Researchers at Deakin University put the number at about a third. They examined the LinkedIn profiles and lobbyist business websites of that third for a study published this year. While they were not able to identify the complete job histories for everyone, their analysis of more than 120 lobbyists showed that 18 per cent had been a member of Parliament or senator, 47 per cent had been a senior adviser or chief of staff and about 30 per cent had been a more junior adviser or media adviser. Most had spent more than 10 years in government before they started working as lobbyists.

"The revolving door between government and industry is common in Australia," writes the report's senior author, Professor Peter Miller from Deakin's School of Psychology.

As of October, there are 884 lobbyists on the federal lobbyist register, from 279 firms, lobbying on behalf of 3691 clients.

There are 227 members of Parliament.

But many people who lobby in Canberra are not required to register as lobbyists. These include charities and religious organisations, not-for-profits, people making representations on behalf of family or friends, or those people engaging in lobbying on their own behalf.

This means unions, or big companies who have in-house lobbyists, do not need to register. People who have worked in state politics also do not have to register on the federal list.

A good - although not perfect - indication of how many people actually lobby is the number of "sponsored" or orange pass holders to Parliament: there are 2380. (There are a total of 12,000 passes issued across various categories in total.)

These are unescorted passes to the private areas of Parliament House, valid for the term of Parliament. To get one, you have to be sponsored by an MP or senator who has known you for at least 12 months. You need to pass a police check and you need to have "a regular and significant business requirement for unescorted access".

The Department of Parliamentary Services notes that some orange pass holders are not lobbyists but others with a legitimate claim to visit Parliament, such as MPs' family members. (Public servants, ministerial advisers and other staff who come to Parliament wear a different-coloured pass and would not be on the orange pass list.)

But the orange pass list is not made public. Parliamentary staff have previously said it would be a security risk to publish it, as people could steal passes or pretend they were a pass holder. Britain and New Zealand are among countries that have published lists of pass holders.

The Rudd government introduced a lobbying code of conduct and a public register of lobbyists in 2008. Anyone who acts on behalf of a third party for the purposes of lobbying government representatives, including ministers, their staff, public servants and Australian Defence Force (ADF) members, must sign up - unless they are exempt.

Registered lobbyists need to confirm or update their details twice a year, which includes the names of people who employ them to lobby. And they must be clear if they have been a "government representative" in the past.

Importantly, the lobbying rules also state that ministers and parliamentary secretaries must wait 18 months before they can start a lobbying career while former ministerial staff, senior public servants and senior ADF members must wait 12 months.

The lobbying code also spells out that lobbyists should not be dodgy in their dealings. Specifically, they "shall not engage in any conduct that is corrupt, dishonest or illegal, or unlawfully cause or threaten any detriment". They should also take reasonable steps to "satisfy themselves of the truth and accuracy" of the information they provide.

We don't have an exact figure - but lobbying is big business. According to think tank The Grattan Institute, US special interests spent about $5 billion on lobbying activities in 2017.

In Australia, private interests donated more than $40 million to political parties at the 2016 federal election, while estimates about the expenditure of major Australian peak bodies and advocacy groups run into the hundreds of millions of dollars.

The Grattan Institute says that in 2015-16 estimates ranged from $400 million to $700 million and did not include in-house or commercial lobbying staff employed by businesses, unions and not-for-profits.

The lobbyist register describes lobbying as a "legitimate activity and an important part of the democratic process". "Lobbyists can help individuals and organisations communicate their views ... and, in doing so, improve outcomes for the individual and the community as a whole."

But there are several major criticisms of the system. One being that the lobbying register doesn't have to include all the people who are actually lobbying.

Many political analysts also express deep misgivings about how the system works, pointing to the "revolving door" between working in government and working in influential industries such as food, gambling and alcohol.

As an unnamed politician explained in the Deakin University study, "Someone retires from politics and then they have a ready-made set of relationships nurtured over many years of being colleagues with other members of Parliament that they can then go and leverage on behalf of a commercial partner."

A former political staffer also told the study: "There's a new minister and I met his chief of staff and like, six months ago, I met him as the industry group lobbyist and now he's working in the minister's office. And so that just amplifies the disparity and the influence between the industry and the community sector, the sort of revolving door."

Critics also point to poor enforcement of the existing regime, questioning how former defence minister Christopher Pyne was able to take a job with consulting firm EY to grow its defence industry business just weeks after he ceased to be a minister.

The Grattan Institute has warned that policymakers who spend more time talking to lobbyists than the general public "may end up with a distorted sense of policy priorities".

The Greens are among those who say significant reforms are needed - and took a lobbying policy to the recent federal election.

"The public rarely knows who is meeting with politicians, for what purpose, and what outcomes are gained," the party says.

The Greens want information about meetings with lobbyists published and a five year "cooling-off" period when ministers and senior staff can't be engaged in private enterprise that raises a conflict of interest or benefit for their past role and connections.

The Grattan Institute similarly wants ministerial diaries to be published and has called for the lobbyist register to be linked to the orange passes.

Lambie also supports the five-year cooling-off period and wants the lobbying code of conduct to apply to anyone who lobbies, regardless of whether they are on the register. She says this should be policed by the Australian Competition and Consumer Commission.

"It will mean anyone who lobbies - whether they're a third-party lobbyist for the mum-and-dad shop down the road, or an in-house lobbyist for Google - has to follow the same standards," she wrote in 2017.

More radical policies are being floated in Washington, where the "revolving door" is on high rotation. Democratic presidential candidates Elizabeth Warren and Michael Bennet want lifetime bans on former members of Congress lobbying legislators.

The very notion prompted commentator Russell Berman, in an article in The Atlantic, to quip, "Good luck with that."