The virus was imported to New Zealand in 1997 to reduce rabbit numbers, but Federated Farmers High-country Section chairman Simon Williamson said most farmers still employed people to shoot rabbits two or three times a year.
''Certainly the virus seems to be working to some extent, but it's certainly not working like it was.
''You have to keep on top of them. They are becoming immune to it in parts of the Mackenzie and Otago, where they are having problems again.''
Mr Williamson said 200 rabbits were recently shot on his own farm, which was between Twizel and Omarama, in just three nights.
Environment Canterbury (Ecan) pest and biodiversity regional manager Graham Sullivan said the council was already undertaking a serological survey of rabbit blood.
''What we're doing there is monitoring the antibody levels in rabbit populations to the calicivirus . . . at several sites.''
Mr Sullivan said although the Mackenzie basin remained ''highly rabbit-prone'', large rabbit populations were now confined to ''large pockets'' of land in the Mackenzie basin and North Canterbury.
''It's not the regional problem that it once was; land use intensification has had a big role in that - large parts of the plains are no longer rabbit-prone.''
Although two wet springs in a row had resulted in lower rabbit numbers this year,
about 75% of rabbits in the Mackenzie Country were now immune to calicivirus, and the serological survey was a way of informing farmers of the situation. Waiting for the virus to control the situation without the aid of additional culling methods ''was not an option'', he said.
Mr Sullivan said Ecan's role was one of regulation and monitoring, and the regional pest management strategy placed obligations on land occupiers to take measures to control rabbits. However,
Ecan found ''high levels'' of compliance among landowners.