The finance minister, Mathias Cormann, has said it was not the government’s policy or aim to achieve a surplus by 2020-21, preparing the ground for an increased deficit or longer timeline to return to surplus in Monday’s midyear budget update.

In an interview on Sky News on Sunday, Cormann refused to recommit the government to returning to budget balance in 2020-21 as projected in the May budget.

Cormann welcomed higher commodity prices but warned they were “not enough” to offset the effect of low wage inflation, falling income tax receipts and low growth in company profits.

Asked to reiterate the government’s policy or aim to return to surplus in 2020-21, he replied: “I think that you’re mischaracterising what the 2020-21 number actually is.

“We’ve always been very clear our objective is to get back to balance, to get back to surplus as soon as possible.

“Based on the best information in May, the economic parameters and policy decisions the government had made at that time, the projection showed the budget was expected to get back to surplus by 2020-21.”

But Cormann refused to pre-empt the updated projection in the midyear economic and fiscal outlook, to be delivered by the treasurer, Scott Morrison, on Monday.

In recent weeks Morrison has also emphasised the 2020-21 figure is only a projection that is subject to change.

Several ratings agencies have warned the government that, if budget savings are stymied, Australia could lose its triple-A credit rating, prompting Morrison to argue debt is only harmful when it is used for recurrent spending in a push to defend Australia’s rating.

In late November, Deloitte Access Economics warned the federal budget deficit was projected to expand by another $24.3bn over the next four years.

Cormann said the budget was in a stronger position than if the government hadn’t made certain savings and revenue decisions, claiming a $250bn improvement over 10 years compared with the trajectory the Coalition came to office in 2013.

Asked about so-called “zombie measures” that had not passed the Senate, Cormann boasted of $22bn of savings achieved since the July 2016 election and said the government would push to pass the balance of the $40bn of measures in the new year.

Cormann said it had projected government spending would fall to 25.2% of gross domestic product but that figure would also be updated on Monday. The government had gone “as hard as we responsibly can” to achieve savings, he said.

Deloitte Access Economics partner Chris Richardson told Sky News that Cormann’s comments appeared to confirm its projections that commodity prices have not been enough to outweigh lost revenue due to low growth in wages and jobs.

Richardson said Australia losing the triple-A rating would not cause an immediate increase in interest rates paid by governments or mortgage holders because of historically low global rates.

“But a few years down the track, when interest rates are higher, it will raise the costs to the federal government, taxpayers, state governments and those who hold mortgages,” he said.

Labor has used the threat that ratings agencies may downgrade Australia’s triple-A rating after the midyear economic update to call for the government to ditch its proposed $48bn company tax cut package.

On Sunday the Greens’ leader, Richard Di Natale, told the ABC’s Weekend Breakfast the government should also consider revenue measures such as axing the diesel fuel rebate and the private health insurance rebate, and making changes to negative gearing and capital gains tax.

“They have got no concrete proposals to address the issue of revenue,” he said. “In fact, their proposal is to give a great big tax cut to corporate Australia at a time when revenues are down.”