Prime Minister Julia Gillard slipped behind opposition rival Tony Abbott as Australia's preferred leader for the first time since August after her credibility was dented when a mining tax she helped design brought in less revenue than forecast.

On the question of who would make the better prime minister, Gillard fell 5 percentage points to 36 percent, while Abbott rose 1 point to 40 percent, according to a Newspoll survey published in the Australian newspaper on Tuesday. The ruling Labor Party rose 1 point to 45 percent on a two-party preferred basis, with the Liberal-National opposition down 1 point to 55 percent, it showed.

Gillard is struggling to build momentum ahead of elections due Sept. 14, amid opposition claims the shortfall in revenue from the tax on iron ore and coal profits is an example of her government's economic incompetence.

Australia's first female prime minister has also been confronted with scandals involving former Labor lawmakers and the resignation of two senior ministers, leading to speculation her predecessor, Kevin Rudd, may make another challenge to her leadership.

In Tuesday's poll, 24 percent of respondents said they would be more likely to vote for Labor if Rudd is restored as party leader, with 13 percent saying they were less likely to and 62 percent saying it would make no difference.

The Feb. 22-24 telephone survey of 1,143 people had a margin of error of plus or minus 3 percentage points. The previous poll was held Feb. 1-3. The two-party preferred measure is designed to gauge which major party is likely to win the seats required to form a government.