The Serious Fraud Office says it will consider a complaint about New Zealand First's handling of donations promptly, because of the legitimate public interest.
But a full investigation will be held only if the SFO believes there is reason to suspect it may find serious or complex fraud.
ACT Party leader Rodney Hide says he laid a complaint because of the Prime Minister's lack of action over serious allegations involving the New Zealand First leader Winston Peters
Mr Peters responded by calling the complaint idiocy, saying no-one takes Mr Hide seriously.
The SFO says it will carry out an assessment before making a decision on whether to investigate.