A new case of Covid-19 in Auckland has cast a cloud over New Zealand and Australia's travel bubble, just one week before it is due to begin.

The new case, revealed yesterday, is a hotel worker at Auckland's Grand Millennium hotel and a close contact of hotel security guard who tested positive last week.

It is the third case at the hotel in recent weeks, after a cleaner tested positive on March 21.

Four locations of interest have been released by the Ministry of Health, including two Auckland bakeries, dairy and phone retailer, but there is believed to be little additional risk to the public.

The development comes just before the start of the long-awaited trans-Tasman travel bubble, with travellers from Australia able to arrive in New Zealand without having to quarantine from April 19.

Prime Minister Jacinda Ardern warned last week the two-way travel bubble could pop should an outbreak occur in either country. Australia briefly suspended the travel arrangement amid a small outbreak in Auckland in February.

However, there is no word from either country that next Monday's opening of the travel bubble will be affected by the new development in Auckland.

Last week, NSW Premier Gladys Berejiklian called for calm after the case of the infected security guard — the second of the three recent cases at the Grand Millennium hotel — was confirmed.

"NSW has always had what I regard as a very balanced approach to borders," she told reporters on Thursday.

"We've kept our borders open the entire time except for a very exceptional case (Victoria's second wave).

"We should accept during this stage of the pandemic that cases will pop up unexpectedly. I would expect others to keep the borders open to NSW if we had an outbreak we could manage, and I extend that same courtesy to New Zealand."

Confirming the start date for the travel bubble last week, Ardern explained how New Zealand authorities would act should an outbreak occur in Australia.

She said cases linked to workers in quarantine facilities were unlikely to disrupt trans-Tasman travel.

"If a case is found that is quite clearly linked to a border worker in a quarantine facility and is well contained, you'll likely see travel continue in the same way as you could see life continue if that happened here in Australia," Ardern said.

"If a case was found that was not clearly linked to the border, and a state responded by a short lockdown to identify more information, we'd likely pause flights from that state in the same way we would stop travel into and out of a region in New Zealand as if it were going into a full lockdown.

"And if we saw multiple cases of unknown origin, we would likely suspend flights for a set period of time."

If an outbreak did occur in Australia, those travelling to, or already in, New Zealand from the affected area may have to monitor for symptoms, take a test, isolate on arrival for a time or, "in some situations", go into 14-day quarantine.

Hypothetically, if Brisbane went into a short lockdown, the New Zealand Government could quickly shut down travel to and from southeast Queensland. But travel to other states, such as New South Wales and Victoria, could continue.