A BENIGN form of calicivirus is immunising rabbits against the disease instead of killing them.

Researchers have discovered that some rabbits in cool, high-rainfall areas carry a benign form of the rabbit haemorrhagic disease.

CSIRO entomology researcher Dr Tanja Strive said the benign strain of the virus did not kill the rabbits, but acted as a natural vaccine.

Dr Strive said researchers had to find the areas in Australia where rabbits were affected, what season the virus was active and at what age rabbits became infected.

Invasive Animal Co-operative Research Centre chief executive officer Professor Tony Peacock said the discovery of the virus had important implications for rabbit control.

He said more research was needed to understand the virus and how it acted so that new strategies could be developed to overcome the problem.

Victorian rabbit expert Steve McPhee said studies of 17 locations across Victoria showed in areas where landowners baited, ripped burrows and fumigated warrens, the numbers had plateaued.

However, in areas where there had been no rabbit management, the numbers had returned to pre-calicivirus levels.

Mr McPhee said farmers needed to be vigilant to keep the numbers down.

"We've now got rabbits where we want them and holding them at low levels requires low effort and low cost," he said.

Lancefield farmer John Blamey said there was a "sea of grey fur" on his property before a co-ordinated program began in the late 1990s.

He said spotlighting showed there were 70 rabbits per km, well above plague levels of 35 per km.

Mr Blamey said the calicivirus and a program of warren ripping and baiting had reduced numbers to about two rabbits per kilometre.