A worker at a New World in Stoke, near Nelson, is one of the recent confirmed Covid-19 cases, with other staff now in isolation and awaiting their own test results.
Nelson-Marlborough Health says it's the region's first case in three weeks.
The woman had returned from the Cook Islands a month ago, but clinical director of public health Dr Stephen Bridgman says that doesn't necessarily mean that's where she contracted the virus.
Travel-related cases can be caused by contact with an infected passenger on a flight, in transit or contact with infected people in another country, he says.
The source of the infection is still being investigated by the Ministry of Health and Nelson-Marlborough Health.
The woman worked at the store in the suburb of Stoke three times being going into isolation, however she wasn't in a customer-facing role and worked mainly outside of the store's opening hours.
Several staff considered close contacts are in isolation and being tested, as well as her household contacts.
The supermarket has also undergone a deep clean.
Dr Bridgman says the risk to other staff and public is low as the woman was wearing a face mask and didn't have a cough.
However anyone with Covid-19 symptoms is still urged to be tested, especially if they were at the Stoke New World between April 18 and 26 and have any symptoms.
Three new confirmed Covid-19 cases were announced today, bringing the total of New Zealand's confirmed and probable cases to 1476.