Federal Labor MP Bill Shorten said forcing out Kevin Rudd as prime minister last week was the hardest decision he made in his political career. He was among a group of leaders who asked Julia Gillard to contest Mr Rudd as Labor head.

Mr Shorten told ABC last night that he had seen the need for leadership change in the two weeks before the challenge. He, however, first approached then Deputy Prime Minister Ms Gillard last Wednesday, the day the challenge became public.

"You couldn't go anywhere without people, good people, people in your own electorate, saying to you 'something's gone really wrong with what the Government's doing'," he said.

"That's why we did it."

Mr Shorten said the public had been critical of the Government's climate change policy and its explanation of the RSPT.

He said the Caucus had to do something to increase Labor party's chance for victory in the next election.

"I suspect for every one of those Caucus members, it was the single hardest decision we have ever made in politics. It wasn't done lightly," he said.

"I concede it was done quickly and I can understand a fair amount of shock, but it wasn't done on the basis of an opinion poll."

The Federal Opposition, meanwhile, has criticised the new prime minister's changes to the frontbench. Ms Gillard gave Simon Crean her portfolios in the Education, Employment and Workplace Relations, while Foreign Affairs Minister Stephen Smith will take on Mr Crean's previous Trade duties. Opposition foreign affairs spokeswoman Julie Bishop said that means Trade has effectively been depreciated in Ms Gillard's Cabinet as Foreign Affairs and Trade should have different ministers.