Former Prime Minister Kevin Rudd has put on the record that he is backing Bill Shorten in the July 2 election.

News Corp Australia publications on Monday reported that Mr Rudd had refused to answer questions on a Shorten government.

Mr Rudd addressed the allegations on Twitter this morning, saying he was "proud" to back Mr Shorten.

Earlier when asked what Mr Shorten could offer Australia as prime minister a spokeswoman for Mr Rudd said he had a "longstanding policy not to comment on Australian domestic politics", The Courier Mail reports.

However, true to form Mr Rudd allows himself policy flexibility, having commented on Australian politics a number of time since he retired from Parliament after losing the election to Mr Abbott in 2013.

Last September when Malcolm Turnbull seized the prime ministership from Mr Abbott, Mr Rudd offered his congratulations.

He later warned Mr Turnbull to watch out for the "nut jobs" on the "lunar right" of the Liberal Party and has called the royal commission into trade union corruption "a piece of politics".

More recently he has praised the inclusion of Pat Dodson in the Senate and the appointment of Stan Grant to the Prime Minister's referendum council.

Meanwhile Mr Rudd's former deputy and later rival Julia Gillard has endorsed Mr Shorten for the top job.

A spokesman for former Prime Minister Gillard said she believed "passionately" that a Shorten prime ministership would be a good thing for Australia.

"In Ms Gillard's view, it is vital all media attention focuses squarely on Bill's plan for a fairer Australia and his policies to put people first," he said.