Prime Minister Jacinda Ardern's trip to the United States has hit another logistical pothole after the RNZAF Boeing carrying the delegation broke down in Washington DC and a third Covid case was confirmed among the travelling group.

Fortunately, the breakdown was on the last stop of the tour - it was due to fly Ardern back to San Francisco for a commercial flight home after her meeting tomorrow with US President Joe Biden.

She will now take a commercial flight instead. The business delegation has already left and most of the PM's delegation planned to catch commercial flights home from San Francisco.

The Boeing - nicknamed Old Faithful Betty within the media - is one of two RNZAF Boeings and there have been famous break-downs in the past, but it did manage to fly the group to the US and then to six cities within the US without problems up until DC.

Bad Betty: The RNZAF Boeing that has broken down in Washington. Photo / Jason Walls

It comes after a third member of Ardern's delegation has tested positive for Covid.

The person returned a positive result this morning. They are asymptomatic and had previously returned negative RAT test results. The person is taking precautionary measures and isolating.

The two earlier cases of Covid-19 in her team - Chris Seed, the head of the Ministry of Foreign Affairs, and Andrew Campbell, Ardern's chief press secretary - both tested positive in San Francisco just before the trip to Washington DC. They are now isolating in San Francisco.

The rest of the delegation, including the Prime Minister and media, had PCR tests earlier today. The media have all tested negative, but have to do a further test ahead of the White House meeting tomorrow.

PM Jacinda Ardern, Clarke Gayford, Trade Minister Damien O'Connor and NZ's Ambassador to the US Rosemary Banks are greeted as they disembark the RNZAF Boeing in Washington DC. Photo / Claire Trevett

Ardern said the absence of Seed and Campbell did not affect the White House meeting – she would usually have been briefed by NZ's US ambassador Rosemary Banks anyway.

"Of course I'm gutted on their behalf, I know they'd been looking forward to the engagement but it doesn't disrupt what would be usual practice for the visit," Ardern said.

There had been "jokes" about whether her partner Clarke Gayford would end up as the note-taker for the meeting but "no, we're not at that point", Ardern said. "At present, we have the same number available to be present at the meeting."

She said there were contingency plans in case of Covid-19 – and the PM herself was pitching in.

Meanwhile, former Prime Minister Sir John Key said Ardern's meeting with Biden will provide an opportunity for them to get to know each other better.

While there could be space for Ardern to raise some specific points, generally these meetings were "choreographed to an inch of its life".

Key told the AM Show that he had met Biden a couple of times and said he was a nice guy.

This morning, Ardern arranged her own press conference with media – a job usually done by Campbell. Ardern said she was "cutting out the middle man" since Campbell was "a bit Covidy at the moment" and it was still very early in San Francisco.

Ardern said she had also now been in contact with director general of health Dr Ashley Bloomfield after learning from the media he had tested positive in Geneva.

She said they had discussed Covid-19 protocols in the countries they were travelling in.

"We had a little discussion about what everyone is expecting over the winter period. So it's fair to say I can tell Dr Bloomfield is well because we immediately got into a bit of a policy chat."

Ardern said such issues were simply part of travelling with Covid-19.