McDouall's first concern was for the person and he hoped they were recovering well, saying it was positive they have not needed to be hospitalised.
But the unknown transmission was of some concern and, as New Zealand prepares to possibly move down to level 3, it was a good reminder to not take level 3 for granted, McDouall said.
"We really, really need to keep our discipline and keep following the rules."
He said on a recent trip to the supermarket he was almost in disbelief when he saw people in front of him shake hands.
"Just stick to the rules because there's no guarantee that level 3 will be announced on Monday and even if it is you have to remember level 3 is no party, it's still extremely restricted."
McDouall said he had heard from a number of people who are keen for the level 4 lockdown to remain.
"It's about everybody, it's about us protecting the vulnerable in the community and we've done it really well so far so let's not take advantage of the last few days."
At the daily coronavirus briefing on Sunday, Director General of Health Dr Ashley 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.
Ongoing discussions were also taking place in Whanganui, where more testing was recently done in response to low per capita testing in the region.
Cases where 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.