Labor will fight five by-elections on its plan to almost double the coalition’s tax cuts, as the government attacks the “Unbelieva-Bill” opposition leader.

Bill Shorten says the upcoming polls, triggered by the dual citizenship crisis, are a choice between the coalition’s corporate tax cuts and Labor’s bigger income tax cuts.

“They are an early opportunity for Australians to judge, to make an early referendum, on what we saw in Tuesday night’s budget,” he told reporters during a media visit to Canberra Hospital on Friday.

He said the Liberals stood for “massive tax cuts to multinationals and big banks”, while Labor wanted a better deal for 10 million working Australians.

The Labor leader used his budget reply speech on Thursday night to announce support for the government’s planned $530 offset for low- and middle-income earners – and then go further.

The $5.8 billion plan will give $350 a year refunds to people earning $25,000, rising to $928 a year for people earning up to $90,000.

Above that, the extra refund tapers down to $140 for people earning $120,000 a year.

Finance Minister Mathias Cormann labelled Mr Shorten a “dishonest populist” and claimed his numbers didn’t add up.

“They’re spending the same money several times over,” Senator Cormann told Sky News.

Senator Cormann argued Mr Shorten had demonstrated dishonesty through a scare campaign about Medicare, his handling of dual citizens in Labor ranks, and ongoing claims about the coalition’s company tax cuts.

Prime Minister Malcolm Turnbull said Australians could not believe “Unbelieva-Bill”.

“He has not produced any of these costings – it is perfectly obvious he has done these numbers on the back of an envelope following Scott Morrison’s budget speech on Tuesday night,” Mr Turnbull told reporters in Sydney.

Asked whether his numbers were rubbery, Mr Shorten said the government was jealous.

“It’s very straightforward. The numbers speak for themselves,” he said.

“You can give $80 billion to large corporations and big banks, if you want. Or you can restore all the sneaky cuts in the fine print of this budget, cuts to hospitals and schools.”

Mr Shorten is banking on Labor’s changes to negative gearing, dividend imputation and capital gains tax to pay for the promises, as well as the decision not to match the government’s corporate tax cuts.

He also promised to return the budget to surplus in 2019/20, the same year as the coalition.