ACT Party leader Rodney Hide says he is considering complaining to the Serious Fraud Office about Winston Peters.
The leader of the New Zealand First Party is refusing to answer questions about the Spencer Trust and any links to his party.
Businessman Sir Robert Jones say he donated $25,000 to the trust before the last election, which he thought was going to New Zealand First.
Prime Minister Helen Clark says she takes Mr Peters at his word and has been given no reason to believe there is any illegal activity.
But Mr Hide says he is looking at whether to ask the SFO, the police or the Auditor-General to investigate the matter.
The issue of donations to New Zealand First is again likely to dominate question time in the House on Wednesday.