AFP have asked the director of public prosecutions to assess the evidence gathered so far

Australian federal police have confirmed they believe at least one offence may have been committed when the media were tipped off about the raid on the Australian Workers’ Union headquarters in October.

The police said in Senate estimates on Friday material had been provided to the Commonwealth director of public prosecutions because they believed the law might have been breached in the incident that triggered the resignation of jobs minister Michaelia Cash’s senior media adviser, David De Garis.

The deputy AFP commissioner, Leanne Close, told estimates the investigation was continuing but confirmed the police had referred material to the DPP seeking advice about whether the law had been breached.

“We believe an offence may have been committed, hence our request to the DPP for an assessment of our material,” she said.

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The AFP commissioner, Andrew Colvin, said the material was not a full brief of evidence but a “pre-brief” seeking “advice on the sufficiency of evidence we have so far”.

Close invoked public interest immunity in declining to say whether the police had recommended charges, how many offences they believed might have been committed, how many people might have committed offences, whether Cash and the former justice minister Michael Keenan had been interviewed or whether the police had ruled out charges against them.

In response to every question, Close said it was “not in the public interest to answer on the ground it may prejudice an ongoing investigation of a possible breach of law”.

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She said warrants had been executed and the police had interviewed “dozens” of witnesses as part of the investigation, which has been running for nine months.

Colvin said the AFP had not briefed the government about the investigation “other than contact that we would ordinarily and normally have as part of the investigation”.

The Labor senator Murray Watt said this would appear to confirm that the police had interviewed ministers, but Close again invoked public interest immunity in declining to answer.

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In October, journalists and television cameras were present when police raided the AWU headquarters as part of the Registered Organisations Commission’s investigation into $100,000 of donations from the union to the campaign group GetUp in 2005.

On 25 October Cash, who was then the employment minister, repeatedly denied that she or her office had been involved, but De Garis resigned after Buzzfeed revealed he had tipped off the media about the raid. The AFP then set up an investigation into the leak.

Labor senators have pursued the AWU raid matter in Senate estimates but have not established the identity of the “media source”, who Cash said had told De Garis about the raids.