Investigators from the Serious Fraud Office have visited businessman Sir Robert Jones to discuss allegations about donations made to New Zealand First.
The ACT Party has complained to the Serious Fraud Office after New Zealand First leader Winston Peters refused to answer questions about party donations.
They include $25,000 Sir Robert Jones says he gave to the Spencer Trust before the last election, which he believed was for New Zealand First.
The trust is administered by lawyer Wayne Peters, who is Winston Peters' brother.
Sir Robert told Radio New Zealand's Sunday programme that he spoke to Serious Fraud Office investigators last week.
But he made it clear he is not pushing the investigation and is annoyed at some reports that he planned to go to the police.
Winston Peters has denied any wrong doing over the donations and has described ACT's complaint as idiocy.
The Serious Fraud Office has said it is carrying out an initial assessment before deciding whether a full investigation is warranted.