Key Points:

New Zealand First has been given a last chance to file accurate donations returns with the Electoral Commission and give a detailed explanation as to why it broke electoral law.

The commission's decision follows hard on the heels of police announcing they would launch a formal inquiry into the party's 2007 donation return.

The commission met this afternoon and considered correspondence from the party about its annual donation returns for 2005, 2006 and 2007.

"The commission is requiring NZ First to produce promised amended party donation returns for the three years and their accompanying auditor's report no later than Tuesday, September 30," the commission said in a statement.

"The commission is unable to release the correspondence or further options for action under consideration, nor comment further."

The statement indicated NZ First either failed to file amended returns, or they were incomplete.

The party has already said it should have declared a $50,000 donation from the Spencer Trust in 2005.

There was some evidence that other donations should have been declared for the 2006 and 2007 year.

Police earlier announced they would investigate a complaint about the 2007 donation, after a complaint from ACT leader Rodney Hide.

The donation return is the only one that can be investigated under electoral law because of the six month statute of limitations.

A police national headquarters spokesman would say only the complaint would be investigated and could not give an indication of how long the inquiry would take.

Mr Hide lodged the complaint last week over an $80,000 donation from the Spencer Trust which was paid to the party in 2007.

The police investigation into Mr Hide's complaint means there are now three separate inquiries into donations to NZ First.

The Serious Fraud Office is investigation donations from Sir Robert Jones and the Vela family, while Parliament's privileges committee is investigating a $100,000 dollar donation from expatriate billionaire Owen Glenn.