Act leader Rodney Hide's canary-yellow jacket has fallen foul of the Electoral Finance Act.
The Electoral Commission wrote to Mr Hide yesterday, saying that under the act the jacket might be an "election advertisement" and therefore required an authorising statement.
Mr Hide said the move showed the absurdity of the act and the Labour-led Government that passed it.
"They have state agents investigating an MP's jacket. They will be checking my underpants next."
Mr Hide got the $1300 jacket in July to be provocative about the act.
He heard nothing until yesterday, when the commission wrote to say it had received a query about the jacket.
The letter quoted the query as saying the garment that "contains the Act logo [and] the slogan 'the guts to do what's right' was worn in public by you in Newmarket and does not contain a promoter statement".
The person who made the query also gave the commission a newspaper article about the jacket and two photos of Mr Hide wearing it.
"On the basis of the information supplied, it appears that the item may be an election advertisement, and if it is an election advertisement, it may not fully comply with these provisions," the commission said.
The act requires "published election advertisements" to carry the authorisation of the party's financial agent - which Mr Hide's jacket does not.
If there is a finding of "illegal practice", Mr Hide could be fined $10,000 and Act's financial agent $40,000.
Mr Hide said he did not know why the commission had taken so long to act, as the query was made in July.
A commission spokesman said the query did not go into its system when made in July and was picked up only when the complainant asked after it.
Mr Hide wore the jacket while campaigning in his Epsom electorate yesterday and said his next step would be "to ignore this for as long as possible".