HOLDING ON: Paula Bennett has held on to the Waitakere seat, so far.

Labour is weighing up legal advice over a challenge in the Waitakere electorate, after it emerged National Cabinet minister Paula Bennett could be tossed out of Parliament if Labour won an electoral petition.

With the Waitakere result hanging on just nine votes, the Electoral Commission has confirmed there are no guarantees that any candidate who loses their seat as the result of an electoral petition would automatically be returned to Parliament off the party list.

But it acknowledges that the outcome is far from certain and the courts could take different views.

Labour will decide today whether to challenge a judicial recount that saw Ms Bennett win Waitakere by just nine votes after a topsy-turvy couple of weeks in which the National Cabinet minister won the seat on election night, lost the seat to Labour's Carmel Sepuloni on special votes, then regain it after the judicial recount.

Labour insiders say a petition is unlikely as the numbers are unlikely to change. The cost is also prohibitive at as much as $50,000.

But Labour has not discounted it entirely and under one scenario raised in blogs yesterday, Ms Bennett would be out of Parliament entirely if Labour won a challenge, because the 2011 general election writ has already been returned.

The Electoral Commission confirmed yesterday that if the court found that the election of a constituency candidate was void, there would be no automatic reallocation of list seats. That would threaten National's majority and would put the number of seats held by it out of kilter with the election night result.

However, the Electoral Commission said the court would also be guided by the need to observe real justice.

"The question would be whether the court was prepared to make additional orders relating to the allocation of list seats when the act makes provision for the allocation of list seats to be challenged by way of a petition to the Court of Appeal," a spokeswoman said.

Ms Bennett said she was not worrying about hypothetical situations. "There are a lot of hypotheticals in this scenario and the advice I've had is that the most extreme outcome is very unlikely.

"But I'm not going to worry about a cluster of hypotheticals. We'll just work our way through things as they come."

Labour has yet to decide whether it would seek an electoral position but was seeking legal advice about several issues, including the reallocation of seats. Leader David Shearer said a petition would probably not change the composition of Parliament.

"We're still talking about that amongst our lawyers and looking at things like cost. Until we actually ascertain the exact nature of it we're not going to make any decisions on that."

The Electoral Commission was also looking into the nine people who voted twice in Waitakere.

Ms Turnbull said that was not an unusually high number. Double votes were usually made by mistake. The commission was yet to decide whether anyone would be referred to police over the matter.