A Labor MP has accused Malcolm Turnbull of using the federal police as his "plaything", following raids on the Australian Workers' Union Sydney and Melbourne offices.
Labor spokesman for employment and workplace relations Brendan O'Connor blasted Tuesday evening's raids, saying the federal government were misusing regulators to attack their political opponents.
"It beggars belief this is not a political raid," he told reporters on Tuesday.
"This grubby Prime Minister is willing to use the police like his plaything, his own personal asset."
The raids "are in support of an ongoing investigation being conducted by the Registered Organisations Commission", the AFP said in a statement on Tuesday.
The raids come after the federal government asked the commission to investigate a donation of union funds to activist group GetUp! when Labor leader Bill Shorten was in charge of the AWU.
Employment Minister Michaelia Cash earlier this month referred the issue to the ROC following reports the union gave about $100,000 to the left-wing advocacy group when it launched in 2005.
The ROC says it had received information that documents relating to the legitimacy of the donations were possibly being interfered with in the union's offices "by being concealed or destroyed."
It said a magistrate authorised the raids to seize the documents.
The AWU says the raids are part of an attempt to smear Mr Shorten.
Union national secretary Daniel Walton on Tuesday said the raids were "an extraordinary abuse of police resources".
Mr Walton said the documents seized were over 10 years old and "do nothing more than highlight the fact that the union made a few political donations."
He said the union's lawyers were looking at legal avenues.
"It is clear the Registered Organisations Commission has been established, not to promote good governance, but to use taxpayer and police resources to muckrake through historic documents in an attempt to find anything that might smear a future Labor prime minister," Mr Walton said in a statement to AAP.
"We're extremely concerned about the actions of the Registered Organisations Commission and extremely concerned of the actions of the Turnbull government.
"This afternoon our lawyers are on the way to the Victorian Magistrates Court to put our case forward so our concerns with how this matter has been progressed and we await to see the outcomes of that."
Senior federal Labor frontbencher Brendan O'Connor said Mr Turnbull and his government were "entirely responsible" for the raids.
GetUp! on Tuesday said it had previously acknowledged receipt of a donation of $100,000 in 2005 from the AWU.
"There is no suggestion of any impropriety on GetUp!'s part on the receipt and handling of the donation," a spokesman told AAP in a statement.
Liberal Senator Eric Abetz said if the GetUp funding was proved inappropriate, it raised serious questions.
"I am pleased that finally issues relating to potential trade union corruption are being taken seriously and thoroughly investigated," he said
"Honest union members have the right to know that their money is being spent correctly."
The commission's website says an ongoing investigation into the AWU's national office and the Victorian branch commenced on October 20.