The Health Ministry's accused of making a major blunder in the advice it gave an Auckland teen, which may result in dozens of other being infected.

The teenager who had contact with a positive case, was told by the Ministry his risk of getting the virus was low, and he didn't need to self-isolate.

He was worried and got a test anyway - and he was positive. But not before he'd interacted with friends assuming he didn't have COVID-19.

Newshub has decided not to identify the 18-year-old, but he's a former student of Pakuranga College who remains mates with many pupils.

On Sunday he was confirmed to have COVID-19.

The school's principal is appalled at how the Health Ministry has handled his case.

"The amount of anxiety and tension it's causing is terrible. I'm very disappointed," Pakuranga College Principal Michael Williams told Newshub.

The teenager had been in contact with a confirmed case - his employer.

But the Health Ministry told the teen he was just a "casual contact", that he was "unlikely to get infected" and he "[did] not need to go into isolation".

The Health Ministry's Director General of Health, Dr Ashley Bloomfield, said he believes Ministry did what it could.