The Mt Roskill byelection is likely to be less than eight weeks away - and the Act Party won't take part in order to give National a better shot at winning.
Prime Minister John Key announced this afternoon that the byelection caused by new Auckland mayor leaving Parliament is likely to be held on December 3.
Key echoed earlier comments by National Party president Peter Goodfellow is claiming underdog status for National.
"No incumbent government has ever won a seat they didn't hold [in a byelection]. So I think we need to accept that is a bit tricky.
"Labour and Greens have done a deal, so that also adds to the complexity and challenge of it.
"That said, we believe we will select a good candidate. We think we've got a good story there...but I think it's got to be seen in the context of [being] quite difficult."
National's likely candidate will be list MP Parmjeet Parmar.
Labour has selected candidate Michael Wood for the seat that has been held by Goff since its creation in 1999.
Roshan Nauhria, leader of the newly-formed People's Party, will also run, primarily on a law and order platform and targeting the roughly 40 per cent of residents who are Asian.
The businessman was one of the founders of the party that launched earlier this year, with the aim of attracting votes from the Indian and other Asian communities.
Act leader David Seymour confirmed this afternoon that he would not stand a candidate, so as to give National the best chance of winning.
"An Act candidate might only get a few hundred or thousand votes. But we want National to win, giving them the numbers to reform the RMA [Resource Management Act] without the Maori Party."
Seymour said he had held not discussions with anyone from National about not standing a candidate and it was the call of Act alone.
The Green Party has 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 Parmar but National got 14,275 party votes - about 2000 more than Labour.
Mt Roskill byelection
• Likely to be held December 3 after current MP Phil Goff was elected mayor of Auckland.
• Electorate includes Auckland suburbs of Mt Roskill, Lynfield, Wesley, Hillsborough, and parts of Three Kings and Sandringham.
• Electorate was formed in 1999, and has been held by Goff ever since.
• According to Census information, about 39 per cent of residents were from the Asian ethnic group - the second-highest percentage of any general electorate.
• The Green Party and Act will not enter candidates.