New Zealand First's Tauranga branch asked the party's seven MPs to vote against the Electoral Finance Bill, a move that would kill the bill.
New Zealand First holds the balance of power on the bill. Without its support it would fail.
A meeting last month of the Tauranga electorate executive, where leader Winston Peters is expected to stand, resolved to ask the caucus not to support the bill but it has not done so.
Mr Peters last night said any disquiet had come from a small group and had arisen before the bill had been reported back from select committee.
Changes had been made to the bill "which we have always promised".
Party activist and executive member Ralph Maxwell said the executive comprised about 18 people. There was one abstention and one vote against the resolution. He said the bill was not supported within the party and that the caucus was supporting it because it would help their own survival.
The bill sets up new rules to regulate election advertising. It broadens the definition of advertising, extends regulation to non-political parties and lengthens the period of regulation to virtually the whole of election year.
Mr Maxwell, 73, is a former Labour MP and minister. He quit the party in 1993 when Mike Moore was deposed.
Asked if he thought New Zealand First's support of the bill would damage Mr Peters' chances of winning in Tauranga, Mr Maxwell said: "It is very hard to fathom what Winston is up to."
Mr Peters is yet to declare his intentions in Tauranga. If New Zealand First falls below 5 per cent in the party vote, winning the Tauranga seat would be crucial to its survival.