South Australia Premier Mike Rann decides to stand down for Jay Weatherill

South Australian Premier Mike Rann will step down from office on October 20, allowing Education Minister Jay Weatherill to replace him.

Mr Rann told reporters on Monday he was putting the interests of the state ahead of those of the Labor Party by leaving earlier than he intended.

He had previously planned to stay on as leader until March next year, mid-way through his government's third term.

The premier said it remained his belief that there was no pressing need to make the transition to a new leader, with more than two years to go to the next state election.

A meeting more than a week ago over the transition arrangements had forced a change, particularly after some details were leaked.

"The people of South Australia elected me and they have a right to know when and why I am stepping down," Mr Rann said.

"So I have therefore determined that in order to complete the job I have to do and to best assist Jay and the government in the leadership transition, that I will step down from the office of premier on the 20th of October."

Mr Rann said he still wanted to finalise negotiations with BHP Billiton over the future expansion of the Olympic Dam copper and uranium mine in the state's north.

A Caucus majority wanted Education Minister Jay Weatherill, who stood beside Mr Rann today as the Premier announced he would move on.

Mr Rann said he had wanted to stay to complete projects - the biggest being the expansion of the BHP Billiton uranium mine at Olympic Dam.

"This is unbelievably complex. It is absolutely important that I do the right thing by the state," he said.

And he made clear he believed his Caucus enemies had broken a previous agreement for him to "mentor" a successor while in office, and then leaked details of the ultimatum delivered two Fridays ago.

"I was disappointed that...inaccurate details of negotiations were leaked in a spirit of obvious mischief and malice," he told reporters.

"It doesn't harm me because ultimately I'll be judged by history on whether out state is in better shape now than when I was elected."

But there was no malice from Mr Weatherill, only praise for the man his backers were in such a hurry for him to replace.

"I've worked with Mike Rann over the last nine and a half years. I've enjoyed every moment of that. I've enjoyed the privileges of being in the port folios he's asked me to serve in, he said.

And I look forward to the insights, the extraordinary insights he no doubt has about this crucial role for our state.