New Zealand First has agreed "after careful deliberation" to cast Jami-Lee Ross' proxy vote while he's away from Parliament.
The Botany MP quit the National Party in spectacular fashion last month and is currently on medical leave.
In a statement, NZ First leader Winston Peters said the party had received a letter from Mr Ross asking it to use his proxy vote.
"The New Zealand First caucus strongly believes that in terms of the Electoral Integrity Amendment Act, that someone in Mr Ross' position should resign his seat and test his mandate with the electors of Botany and we urge him to do so.
"That said, after careful deliberation New Zealand First is able to confirm that it has accepted Mr Ross' request.
"The party wishes to make clear that its decision to accept Mr Ross' proxy, until such time as he revokes it, is done solely on the grounds that the electors of Botany gave a clear expression of their will at the 2017 General Election when they elected him with a significant majority."
While I remain on leave on medical advice, it is important my constituents in the Botany Electorate continue to have a vote cast on their behalf in Parliament. I appreciate NZ First supporting Botany by accepting my request to cast a proxy vote. pic.twitter.com/712dZQjMTY— Jami-Lee Ross (@jamileeross) November 7, 2018
Speaking to reporters at Parliament, Mr Peters said the decision was made in the "spirit of representation" to ensure Botany voters were heard in Parliament.
"We're not here to kick the National Party," Mr Peters said.
"We are here to say to the people of Botany... you deserve to have your voice heard."
Mr Peters said the caucus had agreed unanimously with the decision and would begin casting Mr Ross' proxy vote from 28 November.
National could "get down off its high horse" and use the waka-jumping bill if it wanted to eject Mr Ross from Parliament, Mr Peters said.
Mr Peters said his party had not informed or consulted the Labour Party ahead of making its decision.
"It's for our caucus to make that decision... I wouldn't expect [Labour] to check with us either."
NZ First would cast the vote "at all times in support of the vote of the National Party" as a temporary delegate for Botany voters, Mr Peters said.