The Serious Fraud Office (SFO) has launched an investigation in relation to donations made to the Labour Party in 2017.
In a statement, the SFO said it was conducting four investigations in relation to electoral funding, and a fifth was now before the courts.
"We consider that making the current announcement is consistent with our past practice in this area of electoral investigations and in the public interest," SFO director Julie Read said.
The statement noted that the commencement of an investigation did not indicate guilt, however the director would need to have reasonable grounds to believe a relevant offence may have been committed.
In a statement, the Labour Party said it had not received any specifics into the investigation and it would fully co-operate with any SFO investigations.
It noted despite not being advised on specifics, the party has already made statements to media in February confirming that two men who were then being investigated by the SFO and had made donations to the National Party, had also made donations to Labour.
One donation was in 2017 and another in 2018.
At the time Labour Party president Claire Szabo said both were included as donations in the party return filed in the respective years.
An SFO investigation into the New Zealand First Foundation was launched in February, after police promptly handed in a complaint from the Electoral Commission last November.
The commission said it had formed the view the secretive foundation had received donations that should have been treated as donations to New Zealand First.
It followed reports by RNZ about the way the foundation had been handling donations, and questions about disclosure and donors' identities.
The SFO has said it is on track to decide before September's election whether to lay charges.
The case before the courts related to donations to the National Party, and is set down for trial in September next year.
Former National Party MP Jami-Lee Ross and businessmen Zhang Yikun, Zheng Shijia, Zheng Hengjia were charged with electoral fraud in February after a 10-month Serious Fraud Office investigation.
All four men face charges of deception; Zheng Hengjia has also been charged with providing misleading information.
The scandal traces back to a complaint made by Ross himself when he fell out with his former boss and then-leader of the National Party Simon Bridges in late 2018.
They all pleaded not guilty to the charges in February.
The charges relate to two $100,000 donations paid to the National Party - one in 2017 and the second in 2018.