CANBERRA, (Reuters) – Julia Gillard became  Australia’s first female prime minister today (last night local time) when Kevin  Rudd stepped down, as the Labor government sought to avoid  election defeat later this year by changing leaders.

Gillard is expected to present more of a change of  leadership style than substance, but investors hope she will  soften the government’s controversial “super profits” mining  tax, which is threatening $20 billion worth of investment and  has rattled voters.

“The market is going to assume that the (mining) tax is  going to be amended, and hence the worst case outcome they were  staring at is not going to eventuate,” said Richard Schellbach  an equity strategist at Citi.

Rudd become the shortest-serving Australian prime minister  since 1972, with his leadership falling apart after a string of  poor opinion polls showed him losing ground over recent  decisions to shelve a carbon-reduction scheme and impose a new  mining tax.

Miners have launched multi-million dollar advertisements  warning of widespread job losses, spooking voters, if the 40  percent tax goes ahead in its current form in 2012.

Global miners such as Rio Tinto, BHP Billiton and Xstrata  are expected to campaign strongly against the tax at the next  election and help the conservative opposition’s bid to oust  Labor.

Government lawmakers believe Gillard has a better chance  than Rudd of winning back voters because she is a warmer  personality who can sell policies more effectively.

Despite investor hopes that a leadership change might spell  a major backdown on the mining tax, left-winger Gillard’s  backers expect her to mount a much more effective defence of  it.

But while investors hope Gillard will compromise on the  mining tax, Gillard’s left-wing, trade union background has  some concerned.

“She has been more left-wing than Rudd; she favours more  regulation and spending. So maybe it means the budget deficit  shrinks more slowly than otherwise,” said Su-Lin Ong, senior  economist RBC Capital Markets.