National will need to make history to capture Phil Goff's Mt Roskill seat in an upcoming byelection, the party's president Peter Goodfellow says.

Following his election as Auckland Mayor, Goff will resign from Parliament next week. That will allow for a byelection in Mt Roskill before Christmas.

Goodfellow said National would select a candidate on October 19, with nominations opening next week.

However, the selection of Mt Roskill-based list MP Dr Parmjeet Parmar is all but certain.


Goodfellow said it will be "very difficult" to capture Mt Roskill, as no government had ever won a byelection off the Opposition.

"And of course Labour and the Greens have done a deal for the seat to try and make it even less likely.

"We will however run a strong campaign. Labour has held Mt Roskill since it was created in 1999 and tends to take the electorate a bit for granted."

The Green Party last week announced it would not stand a candidate in the byelection, with co-leader Metiria Turei saying it would be closely contested and the Greens didn't want to "play any role in National winning the seat".

That deal is the first in an electorate under the agreement between Labour and the Greens to campaign together more closely and work to increase the centre-left vote.

It could prove critical - in 2014 Goff won with an 8000 vote majority over National's candidate Parmar but National got 14,275 party votes - about 2000 more than Labour.

Today, Parmar said she was yet to be selected as National's byelection candidate.

However, she had a crack at Labour's candidate Michael Wood, who as the party's candidate in Epsom in 2014 described the deal between National and Act as a "rort".

"The deal that has happened between the Greens and Labour shows that the opposition candidate is not honest about what he stands for.

"I believe in principled leadership. I'm saying this because in Epsom he protested against such a deal, but now he is very willingly accepting such a deal because he believes it will help him get into Parliament."