Another 40,000 chickens will have to be culled due to bird flu at an Otago farm.

It will bring the total number of chickens culled at the Mainland Poultry farm to 200,000 since a high pathogenic strain of avian influenza was detected at the premises last week.

Biosecurity NZ deputy director-general Stuart Anderson said continued testing and monitoring showed no sign of the high pathogenic avian influenza outside of the Hillgrove farm.

The site was under a strict biosecurity lockdown.

"The depopulation to date covers all four layer sheds, which held a capacity of 160,000 chickens in total," Anderson said.

"We've had an animal welfare specialist on site to observe depopulation operations.

"Test results at the weekend confirmed the HPAI H7N6 virus had spread to the final layer shed, which was not unexpected for this virus. We had already planned to depopulate this shed before those results came in as we knew that spread was almost certain," Anderson continued.

"A separate facility on the farm that is away from the layer sheds, and houses 40,000 rearing chickens, will also be cleared. That depopulation will begin today.

"International experience tells us there is no other way to manage the risk of spread of this virus other than full farm depopulation, followed by cleaning, and decontamination."

Anderson said there was no risk to egg and chicken supply in New Zealand "given the size of the national flock".

There were no issues for food safety either and the risk to humans was low, he said.

"We continue to put significant resource into eliminating H7N6, and we are confident we are on the right track to stamp it out."