SYDNEY (AFP) - A defiant Australian Prime Minister Kevin Rudd warned on Monday not to write him off as a new poll showed his popularity slumping barely three weeks away from elections.

After a fortnight of relentless campaigning, a Newspoll for The Australian newspaper showed voter satisfaction with him at 35 percent, down four points, while conservative rival Tony Abbott saw a three-point jump to 41 percent.

The number of people dissatisfied with Mr Rudd rose six points to 54 percent, his worst ever performance as prime minister and just below when he was ousted as Labor leader in 2010 by Ms Julia Gillard.

While he still edges Mr Abbott as preferred prime minister - 43 to 41 - his support is dwindling, having lost three points from the previous poll a week ago.

In contrast, Mr Abbott enjoyed a boost of four points and he is now the closest he has ever been to his rival ahead of the September 7 election.

On a two-party basis, the conservatives are ahead 54 percent to Labor's 46 percent - two percent higher than a week ago.

Despite the dire numbers in a poll of 1,692 voters over the weekend, the embattled Rudd, who re-took the leadership from Gillard in a party coup in June, said it would be a mistake to write him off.

"There are many things worth fighting for in this election campaign. This is a fight worth having," he told Channel Seven.

"If you look at my political career in the past, it hasn't always been easy and I've been written off many, many times before.

"In this campaign, we're still in the second quarter, we're not even up to half-time" he added, likening it to a sports game.

The poll came a day after Mr Rudd warned the country faced the risk of a recession if Mr Abbott was elected, claiming his rival planned to slash A$70 billion (S$81.6 billion) from government spending.

Mr Abbott described the figure as "simply a fantasy", but has not yet revealed his policy costings.

Mr Rudd backed up his attack on the conservative's economic credentials with the release on Monday of a series of adverts accusing Mr Abbott of planning to slash thousands of public service jobs and "cut billions from health and education".

Labor came into the campaign vowing not to be negative and Abbott claimed the ads would backfire.

"He (Rudd) can't open his mouth without attacking the coalition and attacking me," Mr Abbott said.

"That's because they have no record to defend and have nothing positive to say about our future."