Dr Shane Reti's not ready to claim victory in the Whangarei seat just yet.

The National MP - rather surprisingly - is teetering on a knife-edge, with preliminary results giving him a 164 vote lead over Labour's Emily Henderson.

Reti told Kate Hawkesby he knows Henderson could take the win once special votes are counted.

"Where those vote falls, they will fall. At this point in time, I have the victory on the night, which is a privilege, but we will see where those special votes go."

If the seat did turn red, Reti would be able to stay on in parliament as a list MP.

He is currently National's health spokesperson, and is concerned about he quality of Covid-19 pre-testing after 11 international fishermen tested positive and 14 others could also have it.

They came from Russia and the Ukraine on a charter flight and returned positive tests at the Sudima Christchurch Airport hotel on day three of managed isolation.

Reti says this is the largest number of people who've tested positive on one flight, apart from a flight that landed in Auckland on August 23, with 17 positive cases.

He says it gives us reason to pause and look further at pre-departure testing.

"Which is a good policy. This was a charter flight, they didn't have to do it, they choose to, we still think there should be a standard procedure."