Prime Minister and National Party leader John Key is picking David Cunliffe to win the Labour leadership, and says it will be a big step to the left for the party.
Mr Cunliffe, Shane Jones and Grant Robertson attended their last candidate meeting in Christchurch on Tuesday night, and the new leader will be announced on Sunday.
MPs, party members and unions affiliated to Labour have been voting around the country, and Mr Key says Mr Cunliffe is most likely to win.
"It's very important to understand whose voting. It's not New Zealand mums or dads that are voting. It's actually the union movement and they will want payback time when he becomes the leader and that means a big move to the left. And ultimately I think that will have quite a negative impact on jobs and growth for the economy."
Under the Labour party's new rules, unions get only a fifth of vote. The votes will be weighted so the caucus vote will count for 40%, party members 40% and the five affiliated unions 20%.
Mr Key says he will be in Parliament next Tuesday, which will be the first question time for the new Labour leader.
