Defence Minister Marise Payne backed Mr Robert on Tuesday.

"I have complete confidence in the minister and of course the matter has been referred to the Secretary of the Department of Prime Minister and Cabinet,"she said.

But shadow Attorney-General Mark Dreyfus said Mr Robert must explain himself.

"He's got to now make a full statement as to what it was he was doing in Beijing. It can't be both a trip for private purposes and a trip on which he met with a vice-minister in the Chinese government,"Mr Dreyfus told reporters.

He also called on Mr Turnbull to show leadership on the issue:"Running for cover behind a departmental inquiry is simply not good enough."

During a Senate estimates hearing on Monday night,a Department of Prime Minister and Cabinet official revealed Mr Turnbull only requested the investigation at 1.58pm - just two minutes before a fiery question time where the saga featured prominently.

Earlier that night,Mr Turnbull's office distributed a letter Mr Robert wrote to Mr Turnbull. In the letter,the besieged minister noted he was"confident that I have not acted inappropriately"but still asked for the probe by the PM&C.

The official could not tell the estimates hearing how long the inquiry would take.

Opposition Leader Bill Shorten says it isn't looking good for Mr Robert.

"There's a lot of explanation required about what this fellow was doing in China and all the detail of what's gone on,"he told ABC radio.

Mr Robert describes himself a"close personal friend"of Mr Marks,who has donated $2 million to the Liberal Party in recent years.

The scandal comes only weeks after Mr Turnbull lost two ministers - Jamie Briggs and Mal Brough - to scandal.

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