The Otago Regional Council says it is working with Landcare Research to find out why the release of a new strain of rabbit virus has failed to meet expectations.
But Landcare Research rabbit biocontrol initiative project leader Janine Duckworth said the planned natural epidemic after releasing the Korean strain of rabbit calicivirus, RHDV1 K5 (K5), was "basically what we expected to see".
Earlier this week, Cromwell farmer Donald Young called the March release of the virus a failure, and urged the council to take steps to import a strain of virus recently released in Australia, RHDV2.
ORC senior media adviser Emma Schranz said the virus had not had the expected overall impact and "we are working with Landcare to establish why".
"It's not just Otago where the impact has been less than expected."
Dr Duckworth said while the release might not have met individual people's expectations, "to date it's pretty much what we thought might happen."
"If you're looking at a 25% to 40% reduction in rabbits, it's not easy to do that just looking over the fence. That's why you've got to allow us to undertake standardised rabbit counts, monitoring that's planned for the next few weeks and months.
"We were expecting a slow build of infection. This is a naturally spreading epidemic; we are expecting it to build more slowly and rabbits to continue to die for eight weeks or more. That's the message we tried to tell farmers, and some people have either not heard the message or they just really want it to be like it was in 1997."
The Ministry for Primary Industries gave permission to a national rabbit co-ordination group to import the virus in late February, and the council, a member of the group, released the virus from mid-March, at 100 sites across Otago.
Environment Canterbury chief operating officer Nadeine Dommisse said the results in Canterbury appeared to be variable, though the field work for the "detailed post-release study" in Canterbury was not due to begin until the weekend.
Reports from farmers in the Mackenzie Basin were of low numbers of dead rabbits, but the council's Mid Canterbury contractor had reported high numbers of rabbit deaths, Ms Dommisse said.
Dr Duckworth said K5 was imported in order to overcome a benign strain of the virus present in the wild rabbit population which made rabbits immune to the original strain of RHDV1, which was illegally imported in 1997.
It was believed 25% to 40% of rabbits would be susceptible to K5, although the new strain "doesn't kill as well overall," she said.
Landcare Research was working with the council to determine the factors that allowed the virus to work well, but the data it was collecting would not paint a complete picture until August.
Otago Regional Council environmental monitoring and operations director Scott MacLean said there were no plans to import RHDV2.
"Our key messaging is, and has been consistently, that the K5 virus was never going to be a silver bullet and that primary rabbit management needs to be carried out by farmers and other landowners in order to keep rabbit numbers down," he said.