Hawke's Bay farmers are being urged to wait till they are able to buy a pure, more potent strain of rabbit calicivirus disease rather than spreading the weakened form in circulation.

Hawke's Bay Regional Council pest control manager Owen harris said drought conditions and strong sunlight had reduced the potency and longevity of the strains of the virus in New Zealand. He was concerned that rabbits would not be killed by the weakened strains and would instead develop immunity.

"Many South Island farms on which the disease was released last year are experiencing rabbit numbers at the same levels as before the disease was released" Dunedin campany Zentech has applied for registration of a pure strain of the virus and has been granted a temporary licence, Mr Harris said.

A decision on a full licence would not be made till March. If successful, Zentech could then sell the virus. Mr Harris said the disease had shown various degrees of success in different parts of the region. "The first reported release was at Puketitiri in November and we have been monitoring the kill rate and finding difficult to find any rabbits"

An earlier spread at Te Haroto had a kill rate of 80 percent but another at Esk Valley had less than 20 per cent kill despite being spread in perfect conditions, he said.