Rabbit Calicivirus is a viral disease that can cause sudden death in rabbits. It is spread by insects and so even rabbits kept in isolation from other rabbits can catch this disease. Rabbit Calicivirus was released in Australia to aid in control of the feral rabbit population.
There is a vaccine available to prevent this disease and rabbits older than 3 months old should be vaccinated. Unfortunately we cannot vaccinate against the Rabbit Myxomatosis Virus which is also spread by insect bites.
You should house your rabbits in an insect-proof enclosure to prevent this disease as it is regularly seen on the Central Coast.