National could get an extra MP after the death of an Act candidate in the Port Waikato electorate - and that could prove a game-changer in an election shaping up as a close call whether NZ First is needed to govern.

The death of Act’s Neil Christensen just a week before the election has meant the electorate vote in Port Waikato will not count on election day (although the party vote will still count) - and instead a byelection will be held after the election.

The Port Waikato by-election will be held on Saturday 25 November. The by-election Writ Day will be Monday 16 October. The deadline for candidate nominations to be received will be midday Friday 20 October, and the last day for the return of the Writ will be Friday 15 December.

That will mean there are 121 MPs in Parliament after the byelection, instead of the usual 120 MPs.

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Prime Minister Chris Hipkins expressed his condolences to Christensen’s family and loved ones and said he will set a date for the byelection in consultation with other party leaders, once he has considered information provided by the Electoral Commission about possible dates.

Advice from the Electoral Commission is expected shortly, a spokesman for the PM told the Herald, meaning an update could come as early as today.

At 121 seats, a majority of 61 MPs is still needed - so getting that extra MP could help National and Act hit the magic 61 mark needed without calling up Winston Peters.

On the Herald’s Poll of Polls, National and Act currently have 59 MPs between them - two short.

Port Waikato is a safe National seat, held by MP Andrew Bayly. Bayly is high enough on the National list to get in as a list MP on election night.

Christensen was ranked 35 on Act’s list, meaning he was unlikely to become an MP without winning the seat of Port Waikato.

The Electoral Commission confirmed if Bayly subsequently won the byelection, his list spot will go to the next National candidate on the list. That would give National one more seat than it would have won in the election.

“When the preliminary results are announced on election night, and the official results are released on 3 November, they will be based on a 120-seat Parliament with 71 electorate MPs and 49 list MPs (unless there is an overhang),” the Electoral Commission said in a statement.

“When the byelection is held in Port Waikato and an electorate MP is elected, it will increase the size of Parliament to 121 seats.”

In 2015, New Zealand First leader Winston Peters won the Northland byelection and his party got another list MP to take his place.

However, there is also potential for a spanner in the works, depending what happens with Te Pāti Māori. If Te Pāti Māori gets more electorate MPs than its party vote would qualify it for, it could result in a 122- or 123-MP Parliament - and that would mean 62 seats were needed to get a majority. In the Herald’s Poll of Polls, Te Pāti Māori is on 2.8 per cent, and would get four MPs.

National MP Andrew Bayly. Photo / Mark Mitchell

Why it doesn’t necessarily affect the proportionality of Parliament:

Although National would end up with one more MP than it gets on election night, it will not necessarily get an advantage disproportionate to its election result.

Bayly would go from a list MP to an electorate MP and another Nat would come in as an electorate MP - so they would get one more MP than they get on election day.

However, because the Port Waikato seat would not be included in the 120 seats on election day, there would be one more list MP in Parliament than there would otherwise have been - and that will not necessarily be National’s.

It would depend on which list candidate, from any party, was next in line under the Sainte-Lague method, which is used to work out the composition of Parliament based on proportionality under the MMP system. That extra seat could go to any party, depending on the results on the night.

Christensen was born in the UK and grew up in Africa, graduated from Pretoria University in 1977, and relocated to New Zealand in 1984.

He was New Zealand’s only registered specialist poultry veterinarian, according to his biography on Act’s website.

He received the Dennis O’Meara Award in 2017 for contributions to the egg industry.

“I’ve always been interested in the political process and I reckon I keep a fair knowledge,” he said in an interview on The Platform NZ last month.

“On behalf of the Act Party, I’d like to offer my condolences to Neil’s family and friends. Our thoughts are with you,” Act leader David Seymour said in a statement yesterday, adding that the Port Waikato candidate was a valued member of the party.

“I wish to pay tribute to Neil, who was an infectiously charismatic and fascinating man and was New Zealand’s only registered specialist poultry veterinarian. He was a dedicated member of the Act Party who will be immensely missed.”

Bayly said he had known Christensen not just through his standing for Act but also as a poultry veterinarian.

“It’s pretty devastating, I’ve got to say,” he told the Herald.

Bayly described him as an expert in his field, “a real guru”.

Bayly wasn’t concerned about the impending by-election, saying it was not the most important issue right now.

- Additional reporting: Derek Cheng

This story has been updated to reflect it may not affect the proportionality of Parliament.

Claire Trevett is the NZ Herald’s political editor, based at Parliament in Wellington. She started at the Herald in 2003 and joined the Press Gallery team in 2007. She is a life member of the Parliamentary Press Gallery.