The rules for ministers travelling overseas have been expanded to allow the Prime Minister or a minister with a baby to take a nanny or other carer at the taxpayer's cost - but Prime Minister Jacinda Ardern says the taxpayer will not be expected to pay for both her partner and a carer for baby Neve.

Ardern heads to New York with partner Clarke Gayford and Neve on Saturday, but said she was paying for Gayford's flights herself because there were not many engagements for partners on the trip.

"There is no spousal programme for this, so we just made a judgment call that we would cover his travel for this trip. He will be going to some things, but he's primarily travelling to care for Neve."

The Herald has learned that after Ardern became Prime Minister, the guidelines for ministers' overseas travel set by the Cabinet Office were reviewed and changed.

They now allow a minister with a young infant to take someone other than a partner to care for that child or for a minister with a disability to take a support person if needed.

Ardern said she had not sought the change for parents of babies and did not intend to use the extra entitlement herself. She would only allow it for other ministers in "exceptional circumstances."

The Prime Minister signs off on all ministerial travel overseas, other than to Australia, including deciding whether partners can travel with ministers and who pays for them.

Other ministers with young babies include the Green Party's Julie Anne Genter and Education Minister Chris Hipkins, whose partner had a second child this week, who is on parental leave for up to a month.

She said she did not expect to have travel with more than one person, but if there was a situation which required both Gayford and another carer for Neve she would pay for that extra person out of her own pocket.

Ardern said she did not yet know whether she would take Neve to any events in New York.

"We are playing it by ear. There is no set plan, it's just whether or not she's getting enough sleep, where I am for feeds. They might be with us a lot, they might just be in the hotel.

"It depends what the jet lag does to them both. She's a good sleeper and we don't know whether that will mean she ends up sleeping a lot in the day rather than the night."

In New York, Ardern is also staying in apartment-type accommodation rather than the usual hotel because basic kitchen facilities were needed for Neve.

Ardern said it meant she had to be further away from the UN, but she had made sure it did not cost more than was usual.

Accommodation in New York during the Leaders' Week for the UN General Assembly is very costly - most rooms near the UN are more than double their usual rates.

Each Prime Minister sets their own expectations for ministers' overseas travel.

Former Prime Minister John Key always paid for his wife Bronagh's travel and his children when they travelled with him. He had also told his ministers to leave their partners at home or pay for the travel themselves.

His successor Bill English did take wife Mary English overseas with him on the taxpayer.

He said he did not expect his ministers to take partners overseas, but would consider it on a case by case basis. Ardern said she intended to take a similar approach.

The taxpayer did pay for Gayford's travel to Europe for the Commonwealth Heads of Government Meeting in May, but that had included an extensive spousal programme.