The leaders' faces today seemed to reflect their rapidly changing electoral fortunes - with the Opposition Leader sporting a winner's grin in the midst of his 36-hour overnight campaign, and Prime Minister Julia Gillard in a more grim-faced appeal to voters about not allowing a return to Work Choices.

Leading betting agency, Centrebet, has seen three Coalition bets placed for every one for Labor in the past few hours, reflecting voter sentiment with Newspoll showing the government's primary vote plunging to 35 per cent and diving to a tie on 50-50 on the two party preferred vote.

As a result of the plunge, the Coalition has moved from $3.72 overnight to $3 even - and Centrebet's spokesman, Neil Evans, said that was likely to shorten further.

Labor is still a firm favourite to win, easing slightly to $1.36, but this reflected a number of large bets placed in the first weeks of the campaign, Mr Evans said.

"There are a lot of people now clambering to get on the Coalition," Mr Evans said.

Rival agency, Sportingbet Australia, reported a similar overnight trend, with the Coalition firming from $3.60 into $3.15.

Sportingbet Australia's chief executive, Michael Sullivan, said punters had declared the Coalition near-certainties to take marginal seats such as Bennelong, Macarthur and Macquarie in New South Wales and a string of other seats in Queensland and Western Australia.

In Bennelong, Labor's Maxine McKew has drifted to $2.50, but Liberal candidate, John Alexander, has firmed from $2.50 at this time last week into $1.45 favouritism.

The chances of a hung parliament are also firming in the market, with Centrebet's odds going from $3.30 last night to $2.75 today.