Rabbit haemorrhagic disease virus (RHDV), introduced into in Australia and New Zealand as a biological-control agent for wild rabbits, is least efficacious in cool humid areas where a non-pathogenic calicivirus (RCV-A1) also circulates. Heavy rabbit mortality following release of RHDV on cold sub-Antarctic Macquarie Island, where RCV-A1 was apparently absent, not only complemented the planned rabbit eradication operations, especially by reducing secondary poisoning of sea-birds from aerial baiting, but also ruled out cool or humid climate as a major limiting factor of disease spread. In turn, this has advanced the idea that RCV-A1 antibodies inhibit RHDV spread as well as reducing disease severity and mortality.