A new community case of Covid-19 detected in Christchurch shows the testing regime is working but is a constant reminder to keep up vigilance, Prime Minister Jacinda Ardern says.

A staff member working at the Sudima Hotel managed isolation facility at Christchurch Airport returned a positive result on Monday.

International fishing crew members have been staying at the facility. Of the 237 mariners, 31 are infected with Covid-19.

The worker returned a negative test on Thursday 29 October as part of routine testing for staff at the facility, but developed symptoms on Saturday and took another test on Sunday which returned the positive result.

The Ministry of Health said the person is in isolation at home and reported taking care to isolate themselves as soon as they developed symptoms.

The person had visited the Countdown supermarket on Colombo Street on Sunday.

Countdown general manager of corporate affairs Kiri Hannifin said staff were told about the case on Monday night.

She said the store had since been deep cleaned by third-party, professional cleaners.

"As we have done when this has happened, right throughout this year actually, we closed our store down, we did a deep clean. We do all the touch points in the store, we do everything so thoroughly because we don't know where to clean so we just do the whole store."

The supermarket would provide health officials with security footage as part of contact-tracing, she said, though staff were not told of the person's identity.

Prime Minister Jacinda Ardern said the detection of the new community case is a sign the routine testing regime is working.

Ardern told Morning Report the system is built for these types of occurrences.

"MIQ [managed isolation and quarantine] workers are in contact with people with Covid. They are very aware of their circumstances, keep good records of who they're in contact with and behave really responsibly when they feel even a touch unwell, as in this case."

She said the person had only one close household contact, and that work close contacts at the MIQ facility will be re-tested. The Ministry of Health would be putting out push notifications this morning regarding the supermarket visit.

The case was caught early and showed the system was doing what it was designed to do.

"But it is a constant reminder to us that whilst the world is in the midst of significant outbreaks we're not immune to it while we manage New Zealanders returning home. Please keep using the tracer app, please keep up your vigilance, get a test if you're sick."

Low paid workers in isolation facilities must be looked after - union

Unite union president Gerard Hehir said there may have to be an increase in quarantine times for large groups of international workers, and more testing for hotel workers.

He said the arrival of large groups of workers was a different challenge for the isolation facilities than individuals or families.

"Where you've got a large group of people like these fisherman who came in together, possibly in isolation together, or for a longer period of time, that's a different scenario again. It'll need to be watched very carefully.

"We've got to look after those workers ... not just in terms of their health.

"These are low-paid workers, so the risks they're taking, we need to make sure that they are not under pressure to, for instance, not isolate because they might lose money."

He said the workers need to know that the regular testing, which protects everyone, isn't going to lead to them losing work or losing their jobs.

The first of the international mariners were due to complete their managed isolation tomorrow morning. Now they will have to stay on for at least another 24 hours as an additional precautionary measure, the ministry has said.

Those precautionary measures include additional tests - up to four tests for some individuals - and an already lengthened stay in managed isolation.