Peta Credlin says Labor leader Bill Shorten has the potential to be a “formidable” political opponent.

Tony Abbott’s former chief of staff didn’t rule out the possibility of a Labor win when questioned on Sky’s PM Agenda program on Saturday night.

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Ms Credlin said while she was in Mr Abbott’s office Mr Shorten had started to bring unity and discipline to his team.

Mr Shorten has brought unity to his team. AAP

“I thought that in my personal interactions with him one-on-one that he had a likeability, an earthiness and a genuineness that if he was able to work that into his public presentations he’d be formidable,” she said.

When asked whether her former boss had been undermined by colleagues before his exit as PM Ms Credlin said Prime Minister Malcolm Turnbull had always made his ambitions for the top job clear, but now he had a big campaign ahead of him.

“He has my support as a Liberal,” she said.

Mr Abbott last week said he didn’t expect the Liberal Party to ever reverse its decision to vote him out of the top job.

However, he believes what he achieved during his two-year prime ministership will stand the test of time.

“I accept that the party made a decision,” he said.

“I don’t expect the party to ever go back on that decision.”

The former PM says he stuck by his chief of staff and dumped treasurer despite warnings from colleagues because he doesn’t “throw people overboard” to save himself.

Mr Abbott refused to sack Ms Credlin or Joe Hockey – who both also lost their jobs when he was toppled by Mr Turnbull last September.

“When you’re working in harmony with people, I think you sink or swim together,” he said.