The new rabbit haemorrhagic disease virus released in Canterbury and Otago seems to be taking effect.
Environment Canterbury and the Otago Regional Council made controlled releases of the RHDV1 K5 strain in autumn to help reduce the wild rabbit population.
ECan biosecurity regional leader Graham Sullivan said the council was unable to say how many rabbits had been or would be killed.
''However, early results suggest we can expect the reduction to be within the originally anticipated range of up to 40%,'' he said.
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''The virus was released at 92 sites across Canterbury in March-April. The Eastern Mackenzie Basin has the largest rabbit numbers in Canterbury.
''Farmers are advised not to intervene and let the virus spread naturally. They should also undertake traditional control on surviving rabbits as these are likely to be immune to RHDV1.
''Autumn-winter is generally the best time to undertake control, but certainly outside the rabbit breeding season,'' Mr Sullivan said.
Otago Regional Council environmental monitoring and operations director Scott MacLean said about 80% of the sites where the virus was released in that region had shown a decrease in rabbit numbers.
Reductions were in the range of 0% to 80%, the average being 29.9% - in line with expectations.
''We have 10 rabbit samples from the field which have tested positive for RHDV1 K5. This indicates that K5 is active in the wild rabbit population,'' Mr MacLean said.
''If anyone finds a dead rabbit which is not exhibiting signs of an obvious cause of death, it would be hugely appreciated if they contact us to let us know.''