The latest Covid-19 case came from Pakistan via Doha and Sydney.
The woman, who is in her 60s, arrived on June 30 and is in managed isolation at Novotel Christchurch.
As per normal procedure, she was tested on day three of her arrival and the results were negative.
The Ministry of Health says she was tested again "around" day 12 of her stay and the results were positive.
They have not explained why the day of her second test is uncertain.
The running total of cases is now 1,195.
Another patient has since recovered, so the number of active cases remains at 25.