An electoral law expert says Parliament has to sit in late December - whether or not the coalition partners have formed a government.
National's leader Christopher Luxon said agreement had been reached on policy programmes - and now the focus was on ministerial responsibilities.
Wellington barrister and electoral law expert Graeme Edgeler told Checkpoint that Parliament must sit six weeks after the formal announcement of election results.
In the meantime, ministers from the outgoing Labour government have been re-sworn because their warrants expired 28 days after the election, he said.
There was no cut-off as to when a government could be formed, he said.
For example, Spain had elections in July but only worked out who was going to be the new prime minister last week, he said.
"It takes as long as it takes and until at some point someone just decides and realises that we're not going to be able to do then it stops ... they can go for three years if they really wanted to, I very much assume it will not."
All the current caretaker ministers, including Chris Hipkins, were elected so are entitled to be ministers and potentially they could continue in office until the next election although that was extremely unlikely, he said.
"They can be ministers as long as they remain MPs."
Edgeler said MPs who were elected for the first time are being paid now and will receive back pay until the day after the election.
Parliament will sit within six weeks from the day of the return of the writ which means that the latest time it can sit to swear in MPs is a few days before Christmas, he said.
The incoming prime minister can advise the governor-general to summons Parliament sooner than that if a government is formed, he said.
If a new government had not been formed by then the caretaker government would be sworn in again and continue, he said.
"The House would presumably just adjourn to the new year to allow the negotiations to continue."