The case is believed to be connected to the existing Waikato outbreak.
It's not clear if the person usually resides in Whanganui, or was visiting the region when the positive result was returned.
On Tuesday, 90 people in Whanganui presented for a Covid-19 test, slightly down on Monday's 120 tests and Saturday and Sunday's joint total of 145.
Meanwhile, Whanganui delivered it's 100,000th dose on Wednesday. That total is made up of a mix of first, second and booster doses.
According to the latest Ministry of Health data, 46,554 people within the Whanganui DHB region are now fully vaccinated, equal to 81 per cent of the eligible population.
A total of 88 per cent of the eligible population, or a further 4339 people have received at least one dose.
The number of those without a dose of the vaccine continues to drop daily, now sitting at 6234 people, or 10.9 per cent of the eligible population.
Across the wider DHB region which includes most of Ruapehu and Rangitīkei, 324 doses of the Covid-19 vaccine were delivered on Tuesday.