NZ First leader Winston Peters says NZ First may decide not to bring an extra MP into Parliament after his Northland byelection win because his party supports a smaller Parliament.

Mr Peters' 4,000 vote win last night allows him to resign his list position, opening the way for someone else on NZ First's list to enter Parliament.

Mr Peters said the party would "seriously consider" not taking on the extra MP.

"It may be we decide that we are for a much smaller Parliament, that we won't take this option and that we will try and demonstrate that Parliament should be, as the Robertson petition said, no more than 100 people."

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That was a reference to Margaret Robertson's 1999 referendum to reduce Parliament to 99 MPs - a referendum supported by 81.5 per cent of voters. NZ First MP Barbara Stewart also introduced a private members' bill in 2006 to reduce the number of MPs to 100. It passed its first reading but was then defeated after select committee hearings.

Mr Peters said the NZ First board had not yet decided what action to take and there was some time to do so because it would be a fortnight before the final result was announced. Whatever decision it makes will not affect National's ability to get a majority. National has lost an MP so can no longer rely on just Act to get the 61 votes it needs to pass legislation. However, that would be the case whether Parliament had 120 or 121 MPs.

The next on NZ First's list is Ria Bond, a hairdresser from Invercargill who is now working in Parliament as an assistant to two NZ First MPs. If she does not take the seat, Mataroa Paroro is next in line and previously told the Herald he was keen to take it. Ms Bond was not at the byelection function last night, but Mr Paroro was.