Tax breaks used by the wealthy are exacerbating deficits and eroding essential services, while making life harder for ordinary households, according to the ACTU.

The unions' peak body has unveiled its priorities for the federal budget, which include crimping negative gearing and halving the capital gains tax discount, as well as the introduction of a 15 per cent tax on superannuation earnings exceeding $50,000 a year.

The proposals came as Prime Minister Malcolm Turnbull defended his description of negative gearing 11 years ago as a legal form of "tax avoidance".

The ACTU claims $128 billion can be saved over the next decade. Its model is based on ending multinational tax avoidance, scuppering generous tax deductions for the well-off, and introducing a flat minimum income tax rate for the most wealthy to ensure they pay at least some income tax.

"Tax evasion by big corporations plus tax loopholes that advantage the super-wealthy are fuelling inequality and costing our country billions," ACTU president Ged Kearney said.

The ACTU is calling for the introduction of a "Buffet tax", so named because it has been advocated by one of the world's richest men, Warren Buffet, who thought it was immoral that he was required to pay less income tax than his secretary.

According to the ACTU's calculations, that alone would raise an extra $2.5 billion a year.

Speaking to the Canberra press gallery on Friday, Treasurer Scott Morrison acknowledged his first budget was no ordinary affair - occurring just days before Mr Turnbull plans to visit the Governor-General Sir Peter Cosgrove to have both houses of Parliament dissolved for an election on July 2.

He said the blueprint was geared towards rewarding those taking risks, within what he termed "a difficult economy".

While describing his past intention to cut back on "excesses" in negative gearing as "enthusiasms", he hinted that some change may yet be included.

"As we look across the tax system and the way the tax system works, we need to ensure that it is sustainable for the future, and I will be making some further announcements about that next week in the budget," he said.

Any move to narrow the field of negative gearing opportunities for investors would represent another direction change. The government initially flagged taking some action, and then comprehensively walked away from it when Labor put forward a policy of limiting negative gearing to new properties from 2017.

"What I do know, though, is you must preserve the opportunity for Australians to try and get ahead," Mr Morrison said. "The sorts of issues that you refer to or the enthusiasms that I referred to on other occasions are very minimal, very small, and you don't want to do something that puts at risk the broader performance of the Australian property market.

"I think even those who are fans of changes to negative gearing know that Labor's proposal in its composition is confusing and would be disruptive to property markets and undermine the performance of those markets and thereby undermine the value of Australians' homes."

In an interview on Melbourne's 3AW on Friday, Mr Turnbull attempted to explain why he had referred to negative gearing in 2005 as "tax avoidance".

"Well that's taking it out of context ... What I was saying was that the average wage earner, salary earner, has very limited opportunities to reduce their tax ... I go back to my days of law school. Tax avoidance is legal; it may be undesirable and you may want to change the law to avoid it. Tax evasion is what is illegal."

The ACTU's plan involves largely mirroring Labor's policy on negative gearing, limiting it to new properties from July 2017, and halving the capital gains tax discount.

However, its policy on concessionally treated superannuation would tax contributions at the appropriate marginal tax rate "minus a 25 per cent offset", along with taxing super earnings at 15 per cent a year, with all tax rebated for earnings below $50,000 that year.

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