Scott Morrison is selling "H-O-P-E" in Tuesday's budget on the off chance voters are still listening enough to stick with the coalition at the May election.

Two new cancer drugs will be added to the Pharmaceutical Benefits Scheme, while major road and rail projects will be built on the edges of capital cities and the timing of income tax cuts is expected to be brought forward.

The prime minister said getting the budget back to surplus meant being able to afford to make life-saving drugs significantly cheaper.

"The PBS, to me, spells H-O-P-E," Mr Morrison told reporters in Canberra.

"We are in the war against cancer and the fight against cancer. What fuels our army on this is the strong economy and the strong budget."

The first surplus budget in more than a decade will be revealed on Tuesday night, underpinned by strong iron ore and coal prices.

It will include billions of dollars for road projects and freight networks across regional Australia, along with cash payments for power bills.

Finance Minister Mathias Cormann has all but confirmed the budget will include extra personal income tax relief, in line with the coalition's push for lower taxes.

"We've made the decisions in past budgets and budget updates in order to fulfil that commitment (of tax cuts)," he told ABC radio.

"What Australians will see is that we will continue to fulfil that commitment that we've made."

Treasurer Josh Frydenberg's first budget could also be his last, with Labor promising to overwrite it immediately if it wins the May election.

A sceptical Labor leader Bill Shorten stopped short of promising to match any tax cuts in Tuesday's document.

"Will we vote for tax cuts that we haven't seen this week? It depends what they are," he told reporters in Canberra.

"But I also want to put on record, we've already got bigger, fairer tax cuts for 10 million working Australians."

The budget will also include an extra $1 billion for at least seven freight corridors across Queensland, Victoria, NSW and Tasmania.

Mr Frydenberg on Sunday announced $285 million to help almost four million Australian pensioners and veterans - but not people on the dole - cover their energy bills.

The treasurer pushed back against criticism that the government's projected surplus was being built on huge underspending in the National Disability Insurance Scheme.

He argued that wages, which have stagnated in recent years, are picking up compared to the same time last year.

But shadow treasurer Chris Bowen said the coalition had over-estimated the outlook for wages growth in every single budget it had handed down.

Within days of the budget, Mr Morrison is expected to call an election for May 11 or 18.