More than half a million Australians have cast their vote early on the first day of pre-polling while parties scramble to lock in last-minute preference calls.

Around 542,000 voters made their decision across Australia on Tuesday, 70 per cent up on the tally for the first day of pre-polling at the 2022 election, according to Australian Electoral Commission data.

But it was hard to predict whether an expected 50 per cent overall early turnout ahead of May 3 would significantly increase on the back of these numbers, as pre-polling could flatten out during the public holiday period, the commission said.

Australians can vote early in person or by post if unable to do so on election day because of distance, travel, work, illness, caring duties or other reasons.

The coalition announced their defence spending commitment on Wednesday, but might be left without enough time to sell the policy as punters tune out of the campaign.

The defence announcement was better late than never, ex-Scott Morrison media adviser-turned-consultant Andrew Carswell said, as he pointed to conservative voters bleeding to minor parties such as One Nation and Clive Palmer’s Trumpet of Patriots.

“It probably won’t shift the dial but it may bring back some of the support that’s leaking to the right,” he told AAP.

One Nation and the coalition will swap preferences, telling supporters to put the other party second on their how-to-vote cards in key seats.

That included in Mr Dutton’s marginal Queensland electorate of Dickson, where One Nation had him fourth on their preference sheet before reprinting it to put the opposition leader second.

The reprinted how-to-vote cards should be off the press on Wednesday and in the field by that afternoon or by Thursday, although Mr Dutton’s name still appears fourth on the One Nation website as of Wednesday.

“Were reading the tea leaves in a few seats and are conscious One Nation’s preferences will have an impact in this election,” the party’s chief of staff James Ashby said.

“We made it very clear we don’t want another three years of Labor.”

The approach contrasts with Trumpet of Patriots advising people to put Labor and Liberal incumbent MPs last in each seat “to end the two-party duopoly”, which could damage the coalition’s chances in some key electorates.

Asked whether One Nation was worried Mr Palmer’s direction would install Labor MPs in marginal seats held by Liberal incumbents, Mr Ashby said: “I’m not worried, I’m petrified.”

Labor will preference the Greens over the coalition and the Greens will return the favour to keep Mr Dutton out, although there is anger from the minor party over Labor not directing preferences in the key Melbourne seat of Macnamara.

Source: AAP