Colin Barnett said yesterday he would seek an explanation from the State Records Office about a missing file of public documents relating to a union slush fund that Julia Gillard helped set up 20 years ago if he received a formal complaint about it.
The Premier weighed into the saga surrounding the Prime Minister's involvement with a union slush fund in the early 1990s after Federal Liberal Deputy Leader Julie Bishop called for an investigation into the whereabouts of the file and other missing documents relevant to the union's operations.
Fund money was allegedly used fraudulently by Ms Gillard's then-boyfriend, Australian Workers Union official Bruce Wilson.
Paperwork for the creation of an association allegedly used to plunder funds, filed with the WA Commissioner for Corporate Affairs in 1992, should be contained in a State Records Office file, but is reportedly missing.
The AWU Workplace Reform Association's constitution said it was dedicated to workplace safety but in a 1995 interview with her law firm Slater & Gordon, conducted not long before she left the firm, Ms Gillard described it as a "slush fund" for union officials.
There is no evidence Ms Gillard knew about the alleged frauds and she has repeatedly denied any wrongdoing. In August she said she provided legal advice to set up the slush fund but knew nothing about its workings until 1995.
It is unclear when the documents in the file went missing but it is an offence to destroy such records without permission. As well as the empty WA file, Slater & Gordon has revealed it cannot locate its file on the association, which Ms Gillard helped create.
Court files detailing the AWU's discovery of fraud 17 years ago and the fight to overturn redundancies to union officials including Mr Wilson have also vanished.
Ms Bishop said unless there was a plausible explanation for two sets of official documents going missing "you would have to assume" they had been stolen.
Late yesterday, Mr Barnett said: "If a formal complaint comes to me, I will seek some explanation from the State Records Office."
Federal Housing Minister Brendan O'Connor said people were "sick of the smear" in relation to Ms Gillard's role in the AWU saga.