Other countries have a longer history with similar offices: the Netherlands Bureau for Economic Policy Analysis was founded in 1945; the US Congressional Budget Office was legislated into existence in 1974. However, only over the last two decades has the movement to establish independent parliamentary or legislative budget offices taken off worldwide. They are now an accepted part of good budgeting and governance, to the extent that, according to the federal PBO: "At June 2018, 26 out of 35 OECD countries had established an independent fiscal institution. These organisations have a variety of mandates and functions, but they all share an objective of increasing transparency around national budgets."

These offices' roles differ considerably, depending on why they were established. Some prepare official economic forecasts (a role that, in Australia, still rests with the federal and state treasuries). Others have a wide environmental and social reporting remit. Some, like the Netherlands bureau, prepare work not only for parliamentarians but also unions and employer organisations.

By far the largest, and in policy terms most influential, of such bodies is the US CBO. It was created in response to the long and bitter history of conflicts between the executive (that is, the president and administration) and legislative (Congress) branches of the US government. The US Budget Act assumed that politicans would behave more responsibly if they had a better idea of what the budget meant, and established a CBO to serve as the "scorekeeper" for Congress.

Since then, the CBO has become much more than a scorekeeper. It now publishes not only budget forecasts and analyses but also research papers on major economic and budgetary issues facing the United States. It issues an annual economic and budget outlook, a mid-year update to the House and Senate committees, and a long-term budget outlook examining scenarios for the US budget and economy for the next 40 years.

This is a much broader set of responsibilities than any Australian PBO; however, it is important to note that this mandate grew gradually over time, as the CBO became more respected and trusted. Potentially, over time, there will be similar evolutions in Australia.

Here, a PBO's main role is to cost policies for parliamentarians. Australian PBOs differ in who and how they can do this, but all have a costing function as a core part of their business. What does this mean? Costing is done by examining a policy proposal for new spending or taxing, and working out how much that will cost (or raise in revenue) by unpicking its assumptions, testing them, projecting costs over the forward estimates in the budget, and, if the policy, replaces one that is already in the budget, calculating the difference between the existing and the new policy proposal. Costing can be as simple as accepting a number: for example, a policy to provide a grant of $10 million to an organisation in a designated single year costs simply $10 million. In reality, few policies are that simple. Often, costs need to be projected over many years and adjusted for inflation in each year; they may have high start-up costs, which happen only once, or, conversely, start low but ramp up. Working through and "reality testing" the assumptions that lead to different spending or revenue projections in different years might not be easy. Some costings are highly complicated and require detailed economic modelling.

So it is no wonder parliamentarians appreciate help with costings. Of course, a government in power can ask its Treasury officials to do this kind of work, but a PBO levels the playing field, allowing oppositions and (in the case of the Commonwealth and Victoria) minor parties to have their policies costed.

Just as importantly, arguably more so, a PBO's role is to make the costs of policies transparent: publish the results so the public can see what the proposals will do to the budget bottom line. In the case of NSW, the PBO adds up the proposals from both government and opposition and publishes a budget impact statement for both sides five days before the election – so the voters can see exactly what the impact on the budget will be before they cast their vote.

In NSW, the PBO is intermittent, focusing on costing policies for the election. It is established from September in the year before the state election (which, in 2019, will be on March 23) and winds up by June 30 in the election year. That is, of course, only possible because NSW has fixed-term elections.

The federal PBO is an ongoing office, and provides policy costings and more general briefings to all parliamentarians. Victoria's PBO will also be ongoing.

The NSW arrangements also differ from those in the Commonwealth in that the NSW PBO can only cost policies proposed by the premier and the opposition leader. Neither minor parties nor individual MPs are allowed to submit policies for costing. The federal PBO, by contrast, is able to provide costings for any parliamentarian. Its published costings on its website are predominantly from Greens, independents and minor parties. It does, however, prepare post-election reports showing costings for all parliamentary parties.

The NSW PBO Act was amended last week to shorten turnaround times for requests for information, require the NSW Treasury to prepare a pre-election budget update, and incorporate a range of other minor improvements to PBO operations. During the debate, Labor reiterated its policy that the NSW PBO should be ongoing, to provide further support to the Parliament outside the immediate pre-election period. This was supported by the Greens, who also argued strongly, for obvious reasons, for the PBO's remit to be extended to doing work for parties other than the government and opposition.

A PBO helps develop and maintain public trust in political processes. It remains to be seen whether states other than Victoria will establish their own PBOs in the future. Nevertheless, they are important. A PBO that works well will improve trust in government, allow better policies to be developed and contribute to good governance. There is a decline in trust in governments worldwide – while it is not everywhere and not uniform, it is a worrying development. Institutions like PBOs play an important part in counteracting that trend.

Stephen Bartos was just reappointed as NSW's Parliamentary Budget Officer and will be in the role until the end of June 2019. This will be his last Informant column until his term ends.