Whanganui District Health Board chief executive Russell Simpson said the person was not an essential worker and the public health team was working hard on contact tracing.
All other people in the person's household and bubble were tested on April 17 and their swab results are still pending.
Simpson said the DHB, through the medical officer of health and the Ministry of Health public unit, is exploring options for community testing.
Director-general of Health Dr Ashley Bloomfield recently advised the Whanganui DHB to increase its testing.
Since then the DHB has done more testing, with 629 people being checked from a total of 1582 assessed.
Simpson said over the last four days, a rolling average of 71 per cent of people presenting at a community-based assessment centre have been tested.
"WDHB encourages all those who have a cough, cold, running nose, sore throat to come to one of the CBACs for assessment," he said.
At the daily coronavirus briefing on April 19, Bloomfield said there were only six recent cases where the source of infection was unknown.
The cases include the one in Whanganui and one in Timaru. Targeted community testing had already been carried out in Timaru to gauge the prevalence of Covid-19 in that community.
Cases in which the source of infection was not known were of higher concern and Bloomfield said thorough tracing of contacts and workplaces was being undertaken by the public health units in areas where those cases were.
The seven other people in the region with Covid-19 have now all recovered.