Senior Liberal Arthur Sinodinos' failure to declare six directorships is a 'very murky matter', the prime minister believes.
Senator Sinodinos admitted to parliament on Thursday that he had been forced to update his pecuniary interests register with six directorships, following inquiries from the media.
He described the failure as an innocent oversight but Ms Gillard believes the matter is more serious.
'This is a very murky matter,' she told reporters in Tasmania on Friday.
'It's not one for me, it's a question of leadership for Mr Abbott and how he will respond.'
Opposition Leader Tony Abbott conceded Senator Sinodinos should have declared his pecuniary interests earlier.
'Obviously, there are rules and people should comply with the rules,' Mr Abbott told reporters in Brisbane.
But he rejected a suggestion his colleague was tainted by his involvement with a company linked to Labor powerbroker Eddie Obeid, who recently appeared before the NSW Independent Commission Against Corruption for allegedly trying to influence a coal mine approval.
'There's a world of difference between exploiting an official position for personal gain and inadvertently overlooking to declare a couple of not-for-profit directorships,' Mr Abbott said.
Finance Minister Penny Wong said she could not understand how Senator Sinodinos could forget six directorships.
'It doesn't really pass the person-in-the-street test,' she told Sky News, pointing out that Senator Sinodinos wasn't new to parliamentary processes, because he served as chief of staff to former prime minister John Howard.
He is also a former NSW Liberal Party president.
Senator Sinodinos also confirmed to the Senate he had abandoned a claim to a five per cent shareholding in Australian Water Holdings, the company linked to Mr Obeid.
He says he only became aware of the Obeid family link after he joined the company and had been 'shocked and disappointed' when he found out about the connection. - AAP