NSW Labor has referred the alleged leaking of former leader Michael Daley's private information to the state's corruption watchdog for investigation.
Mr Daley in February contacted police after The Sydney Morning Herald reported that his office had used an MPs hotline to contact Revenue NSW about a speeding fine in his name.
The then-Labor leader, who insisted his wife had been driving the car at the time, believed it was a breach of privacy.
It later emerged that staff working in then-finance minister Victor Dominello's office were interviewed as part of the investigation.
NSW Police on Tuesday said their inquiries had concluded and no criminal charges had been laid.
"In the absence of other evidence no further investigation is possible," a spokesperson said in a statement.
But Labor has now referred the alleged data breach to the NSW Independent Commission Against Corruption.
"The referral is in relation to a file that was obtained by staff in Minister Dominello's office from Revenue NSW," interim leader Penny Sharpe said in a statement on Friday.
"The file contained the private personal information of hundreds of NSW citizens."
Labor claims the file ended up in the hands of the Liberal party during the state election campaign, which is not allowed under NSW privacy laws.
"Minister Dominello has been unable to confirm whether this data has been deleted, or provide any information about what he knew about the treatment of this information," Ms Sharpe said.
"He has also refused to inform those citizens whose data has been breached."
ICAC is able to subpoena evidence and compel witness statements - unlike police, who were restricted by the Privacy Act during their investigation, she said.
Mr Dominello, who's now the customer service minister, told parliament on Tuesday his staff were "asked to assist the relevant authority in its investigation" and he instructed them to do so.
He acknowledged the police statement and said he had nothing further to add.
When asked later why an advisor resigned from his office, Mr Dominello simply confirmed the staffer had resigned and he'd accepted the resignation.
Premier Gladys Berejiklian was asked if her chief of staff had known that the Herald's story was based on information "disseminated without consent".
"To the best of my understanding, not a single member of my staff was contacted by police or interviewed and the matter is closed," the Liberal leader told parliament.