A senior advisor to Bill Shorten has stepped down in the second top-level resignation from his office within months.

The Opposition Leader’s Deputy chief of staff Sarah Adams will leave, after she was overlooked for the top job in favour of Queensland Labor Party Secretary Cameron Milne, the Nine Network reported on Sunday night.

Labor MPs told of her departure said they believed Ms Adams should have been promoted to the role and that it was a “mistake”, calling Mr Milne “a party hack”.

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Ms Adams temporarily ­replaced Mr Shorten’s chief of staff Ken McPherson who resigned, ­reportedly for ill health, in June.

Since then Mr Shorten had struggled to fill the role.

“She was smart and capable, and worked bloody hard in the acting role as chief of staff,” one senior Labor MP told News Corp.

Pressure is mounting for the PM. Photo: Getty

Government sources, who had learnt of Ms Adams’ departure, said it was “a bit rich” that so much ­attention was paid to the Prime Minister’s office when Mr Shorten’s was in chaos.

“He has lost his chief of staff and deputy in the space of three months. Tony has had the same chief of staff for more than three years,” they said.

“Not to mention that he has overlooked a capable woman for a party hack.”

Meanwhile, Coalition frontbencher Malcolm Turnbull refused to rule out to colleagues that he would challenge Prime Minister Tony Abbott for the leadership, the Nine Network revealed on Sunday night.

The ABC reported that they had spoken to eight government ministers, with six saying the Prime Minister’s leadership would face another challenge before the end of the year.

Mr Abbott said he was more concerned about running the country than the latest leadership gossip, a week out from the crucial Canning by-election.

“I’m concerned with good government,” he said.

“That’s what I’m preoccupied with every day, not insider Canberra gossip,” he told reporters in Perth on Sunday.

Senior government frontbencher Peter Dutton said Mr Abbott has strong support in the party room.