Foreign aid spending will take a multi-billion hit under a Coalition government, according to the long-awaited final policy costings and budget cuts announced this afternoon by the Opposition.

Treasury spokesman Joe Hockey says the cuts are necessary to "grow" Australia's economy.

"We can only be a more generous nation to the rest of the world if we have a strong Australian economy," he said.

"And so we are reducing the growth in foreign aid by $4.5 billion over the forward estimates to fund essential infrastructure here in Australia."

The Opposition says the money will be spent on infrastructure projects, including $1.5 billion on Melbourne's East West Link, $1.5 billion on Sydney's WestConnex and another $1 billion on an upgrade to Brisbane's Gateway Motorway.

"You'll see that we're front-end loading a lot of the infrastructure spend so we get projects up and away," finance spokesman Andrew Robb said.

Growth in foreign aid spending will also be tied to the Consumer Price Index.

UNICEF Australia has reacted angrily to the announcement.

"Mr Hockey may well wish to argue the economy will grow faster under a Coalition but his costings are at the expense of children's lives," UNICEF Australia CEO Norman Gillespie said in a media statement.

World Vision Australia chief Tim Costello says the announcement is "truly devastating".

Key points: Coalition says its figures would improve budget bottom line by $6 billion.

Coalition says its figures would improve budget bottom line by $6 billion. Extra $1 billion would be seen as a result of an increased economic activity after scrapping of carbon tax.

Extra $1 billion would be seen as a result of an increased economic activity after scrapping of carbon tax. A reduction in government debt by $16 billion.

A reduction in government debt by $16 billion. Extra $5 billion over forward estimates for key infrastructure projects.

Extra $5 billion over forward estimates for key infrastructure projects. Infrastructure projects to be paid for by $4.5 billion cut to foreign aid.

Infrastructure projects to be paid for by $4.5 billion cut to foreign aid. Re-phase $650 million of Murray-Darling Basin water buy-back programs.

"We understand our country faces economic challenges but we should never, ever balance the books on the backs of the poor," he said in a statement.

Greens leader Christine Milne also lashed out at the decision, saying supporting infrastructure "at the expense of the world's poorest people is shocking to behold".

Other "modest" savings will be made through spending $650 million on the Murray-Darling Water Buyback Scheme over six years, instead of over four.

The public service efficiency dividend will also be increased by a further 0.25 per cent, by targeting government advertising, consultancies and travel. The Coalition forecasts that will save more than $400 million over the forward estimates.

Mr Hockey says there will be no cuts to education, health and defence spending.

However, the Coalition expects to save $38 million over the forward estimates by reversing Labor's promised additional funding for the Australian Curriculum, Assessment and Reporting Authority.

Meanwhile, $5 million would be saved by scrapping the planned development of an Australian Baccalaureate, and $10 million would be redirected from the Better Schools program to the Coalition's Secure Schools policy.

Albanese says costings a 'farce', attacks late release

The Coalition says the additional savings bring the total amount of cuts to approximately $40 billion.

Deputy Prime Minister Anthony Albanese has dismissed the figures as a "farce".

"They've been hiding their costings and we know why. There's some shockers in this document," he told Sky News.

"You don't know what the detail of the policy that is there is, and how it matches up with the figures."

The Labor Party has also attacked the Coalition for releasing the details so late in the campaign - and after the broadcast blackout for campaign ads kicked in midnight last night.

According to the Opposition, the Parliamentary Budget Office says the budget will also improve by $1.1 billion as a result of boosted growth following the abolition of the carbon tax.

Senator Milne has challenged that figure, saying "abandoning commitment on climate change" will prove costly.

"Over the last six years, the cost of extreme weather events to the federal budget has been $6.7 billion. When you add in the costs to local and state government and insurance losses, it goes up to $10 billion," she said.

Hockey says budget $6 billion better off

The Coalition also expects to earn $93 million this year through a "stopping the boats dividend", which it says will grow to a total of $1.1 billion over the next four years.

That will be balanced by $75 million worth of costs this year by keeping the fringe benefit tax exemption on company cars. That cost will grow to $1.9 billion over the next four years.

It says its 1.5 per cent levy on big business to cover the paid parental leave scheme will earn $300 million in each of the first two years of operation. It is tipped to increase to $3.8 billion in 2016-17, though it is unclear what is behind the revenue jump.

Mr Hockey says the Coalition's measures will improve the budget bottom line by $6 billion and reduce government debt by $16 billion.

"That's $6 billion of real money and taxpayers' money," Mr Hockey said.

"I am leaving our budget numbers in better shape than what we're spending."

However, the details do not include a projected deficit or surplus position, as foreshadowed by the Opposition.

"The Government might be in deficit for nearly a decade in the scheme of things, as a result of where we stand on the budget," HSBC's chief economist Paul Bloxham said.

"I don't think today's numbers necessarily add a whole lot more to that. But on the face of them, the numbers suggest a little bit more savings."

Parties have 'acted fairly responsibly' with costings

Macroeconomics director of budget and forecasting Stephen Anthony says both major parties appear to have released costings that are broadly credible and have offset spending measures with savings during the campaign period.

"It is possible to nit-pick about assumptions but, overall, at least in terms of the overarching impact on the budget bottom line, both sides have acted fairly responsibly," he said.

However, Mr Anthony says both major parties score a fail when it comes to taking serious savings or revenue measures that would ensure a return to balanced budgets in the longer term.

He says both parties' forecast return to surplus in 2016-17 is based on what he feels are optimistic Treasury assumptions of a fairly rapid return to average growth levels.

"Based on the continuation of sub-trend growth over the next few years, and declining terms of trade, we would anticipate that over the forward estimates period whoever is in government will run deficits of over $100 billion," he said.

"By the end of the 2016-17 [financial] year, we're looking at another deficit of something in the order of $30 billion."

Mr Anthony warns that longer term policy proposals such as Disability Care, the Gonski education reforms and the Coalition's commitment to extra defence spending are likely to keep the budget in a structural deficit well into the next decade.

This would result in net Commonwealth debt that could reach 15 per cent of GDP - a level that will start to worry financial markets and ratings agencies.

"The only thing that will save us is growth returning to trend or above trend and/or the terms of trade rapidly improving again," he cautioned.

"That's counting on windfalls again - more sunshine less rain - and that's not really prudent budgeting."