Scott Morrison has refused to guarantee that the government will run budget surpluses of at least 1% of GDP, leaving room open to grant larger personal income tax cuts in the May budget.

With a dramatic improvement in revenues allowing the government to increase infrastructure spending and ditch the planned $8bn Medicare levy increase, the Labor leader, Bill Shorten, has accused the government of attempting a “desperate pre-election cash splash”.

At a doorstop in Canberra on Monday, Morrison said the budget would be “very responsible” and the government had been “very cautious in our forecast”.

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Deloitte Access economics’ budget monitor found that the company tax take has risen by $36.2bn since this time last year and income tax is $10.6bn higher, causing the rolling annual cash deficit to shrink by $15.4bn in the last four months alone.

Asked if the government would stick to its commitment of budget surpluses of at least 1% of GDP in the medium term, the treasurer said the budget’s trajectory would be set by “ensuring taxes do not rise more than 23.9% of the economy”.

“Ensuring that you have a guard rail on how much you tax and having a clear understanding of how much you can spend, that is how you manage a budget and stay in the middle,” he said.

While the government still plans to return to surplus in 2020-21, Morrison made no commitment as to the size of planned surpluses, promising only that 10-year projections would be included in the 8 May budget.

Morrison warned that Labor’s pledges at the 2016 election would have resulted in taxes rising to 25.7% of the economy, the highest in Australian history.

In addition to confirming the Coalition’s commitment to the second tranche of the 10-year company tax package, the budget is expected to contain income tax cuts which the Australian Financial Review has reported will start slow and rise over the decade.

Morrison was also asked if the government had discussed holding the superannuation guarantee at 9.5% rather than allowing it to rise to 12%.

Both the revenue and services minister, Kelly O’Dwyer and the Grattan Institute warned that raising it to 12% would hurt wages and could also hurt low-income earners’ retirement savings.

Morrison deflected the question, saying he would not go into deliberations of the cabinet or government. He said the superannuation guarantee “hasn’t been an issue that we have been proposing any changes to”, without stating what level was preferred.

In a statement the shadow treasurer, Chris Bowen, said Morrison’s remarks suggested the Turnbull government had “waved the white flag on budget repair”.

“As [Deloitte Access Economics’] Chris Richardson says, the rivers of gold are running again and yet we are set to see another budget where a 1% of GDP surplus will be nowhere to be seen,” he said.

With the Coalition seeking to cast Labor as the party of high taxes at the next election, Shorten has has sought to set the test of the 2018 budget as its commitment to funding services.

Federal budget $7bn better than forecast as 'rivers of gold' flow, Deloitte predicts Read more

At a speech to the McKell Institute in Sydney, the opposition leader said there had been a a “marvellous surge in revenue, a tremendous improvement to the bottom line, just in the nick of time for a desperate pre-election cash splash”.

He said what really counted was “not what the budget does for the job security of a prime minister and a treasurer ... it’s what a budget does for your family and for our nation”.

“Does it invest in your child’s school? Your daughter’s university? Your son’s Tafe? Does it guarantee that you can afford to see a doctor when your loved one needs to?”

Shorten said Labor was committed to social spending because there was “a dividend from this investment in our fellow Australians”.

Labor planned to pay for these commitments by targeting tax concessions such as negative gearing, which redistributed national income against the interests of the less well off, he said.