Malcolm Turnbull has said, for the first time, how the Coalition will pay for some of its $50bn tax cut plan.

It will partly fund the cuts with “substantial savings” on superannuation tax concessions in his budget, the prime minister says.

The budget shows that the 10 separate items in the Coalition’s superannuation reform package will raise about $6bn over four years, and lose about $3bn. It does not provide 10-year estimates.

Turnbull has also dismissed a letter from the former prime minister Paul Keating, published in the Australian Financial Review on Friday, in which Keating flatly rejects Coalition claims that his company tax cuts in the 80s and 90s showed the wisdom of its current policy.

Keating said he paid for vast reductions in company tax by “a massive broadening to the base of the tax system”.

That included capital gains taxation at full marginal rates, a fringe benefits tax, the abolition of entertainment as a deduction and tax on company cars.

“I would never have countenanced a $50bn impost on the budget balance with a discretionary unfunded tax cut,” Keating wrote.

He described the Coalition’s proposed tax cut plan as a “massive impost on the national fiscal balance”.

But Turnbull said Keating’s intervention in the campaign, at this late stage, should be taken for what it is – a political intervention designed to support Bill Shorten’s Labor.

He said the Coalition would be able to pay for the tax cuts, in part, with savings from super.

“I would say this with Paul Keating, knowing and respecting him well, I would look at what he did in office as a measure of his commitment rather than what he writes in a letter to a newspaper a week out from an election,” Turnbull said on Friday.

“It will not have escaped your notice that we have made very substantial savings in respect of superannuation and so we have made savings in tax and that contributes to the cost of the – the notional cost anyway – of the company tax cuts,” he said.

Shadow Treasurer Chris Bowen has jumped on Turnbull’s claim, saying the Coalition’s tax cuts will cost far more than any savings made from super.

“Malcolm Turnbull attempted today to say that the corporate tax cuts were paid for by changes to superannuation. I mean that is just a joke?” Bowen said.

“Is he really now asserting to the Australian people that his superannuation changes make $50bn over the next decade? We all know that’s not the case.”

As treasurer and then prime minister, Keating cut the company tax rate from 49% to 33%, including a cut in 1993 when the budget was in deficit.

Turnbull had used Keating’s cuts to justify the Coalition’s proposed $50bn company tax cut on numerous occasions, including during Q&A on Monday.

He said: “The biggest cutter of company tax in our lifetimes is in fact Paul Keating, who cut company tax twice and he cut it because he knew that if you reduced company tax, if you reduced business tax, you increase the return on investment.”

In the Facebook leaders’ debate on 17 June, Turnbull said: “Paul Keating was right when he cut company tax.”

On 8 June, Turnbull said “the bulk of the benefits from a company tax cut go to employees, go to labour”.

“That is accepted by economists,” he said. “That used to be accepted by the Labor party, it [was] certainly accepted by Paul Keating.”

Keating accused the Australian Financial Review of a “campaign in collusion with the Business Council of Australia” in the council’s “camouflaged attempt to reduce the rate of company tax on foreign shareholdings”.

On Thursday, Bill Shorten told ABC’s 7.30: “You can only afford to reduce taxes when you replace them with other taxes. But Mr Turnbull keeps trying to pretend he’s some sort of poor man’s Paul Keating. He’s not.

“Mr Turnbull’s only picked up half of the lesson from the Labor party; you can only abolish taxes or reduce taxes when you’ve got a source of income to replace them.”

On Friday, the Coalition’s campaign spokesman, Mathias Cormann, told Sky news that voters should look at what Keating did, not what he now said.

“He’s a Labor man, he’s supporting Bill Shorten in this campaign, so that is hardly a news flash,” the finance minister said.

The company tax cut in 1993 came when the budget was in deficit, Cormann said. He also disputed Keating’s claim that he had paid for his company tax cut with a broadening of the tax base. “I don’t that’s actually quite right.”

He added: “Every credible economist will tell you a cut in company tax will help you boost investment, boost productivity, boost growth, and create more jobs and increase real wages over time.”