In the first indication the K5 virus is doing its job, an early report shows a 25 per cent decrease in rabbit numbers in Otago.

However, Federated Farmers says the results are "disappointing'', and a similar decrease may have been achieved through regular culling methods.

An initial report by the Otago Regional Council released yesterday shows the impact of the RHDV1 K5 virus across 30 out of the 94 sites where it has been released.

It shows a total of 4686 rabbits decreasing to 3520, which is a 24.9 per cent drop.

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In a release yesterday the council averaged the percentage change across the 30 sites, which shows a 29.9 per cent decrease.

Population reductions ranged from 0 per cent to 80 per cent, while some sites had population increases ranging from 7 per cent to 45 per cent.

Eleven of the 30 sites had deductions of more than 60 per cent.

The sites were in the areas of Roxburgh/Teviot Valley, Wanaka/Tarras, Cromwell/Queenstown and North Otago.

The numbers were collected through "night counts''.

Regional council environmental monitoring and operations director Scott MacLean said these counts provided only a snapshot of one particular moment in time, and therefore accuracy was "difficult to establish''.

"We also conduct an annual count, but that generally gives a different data set.''

The council was hoping to have a full count soon, although it was delayed due to weather issues.

Any increase in virus-related mortality was a positive, as it meant the virus was working and established in the population, MacLean said.

The statistics of overall reductions were in line with the council's expectations, he said.

Increases in rabbit numbers in some areas were likely due to background immunities, which the council anticipated, he said.

"There were no surprises. However, all this does is reinforce the importance of conducting primary controls as has been part of our key messaging related to the virus release.''

The council had 10 rabbit samples from the field which tested positive for the virus.

"This indicates that K5 is active in the wild rabbit population.

"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.''

The virus was released in Otago in March.

Otago Federated Farmers president Simon Davies said general consensus was farmers were "disappointed" with the results of the virus.

"They were expecting a better kill than perhaps what was achieved. Some farmers who have a bigger issue have not seen a significant drop at all.''

While a 25 per cent decrease overall sounded good, very few did their own control this year to allow the virus to spread, he said.

"For a number of farmers they may well have managed a 25 per cent decrease on their own and they're now really lining up for next spring for an explosion.''