From what I read the virus transfers from one place to other place, at beginning when there is no symptoms tests can be done in mouth or throat or nose or eyes where is infected, after symptoms shows the virus moves down to the body and will be negative if tests is done in mouth/throat/nose/eyes. That's why China is doing more than 1 tests on people after things out of control. But these is not from official news or heard from a doctor myself, just rumors on the internet.
Thanks. That makes sense. Maybe that is why NZs first case initially tested negative on the 1st two tests, and it wasn't until the final test which I understand is taken further into the body , did they test positive, as they were apparently at that stage already showing symptoms .