For those who may be interested in searching for the numerous Cornish families who emigrated to Australia, many of the state BDM registries are available online. (These are typically the same or similar index records that can also be purchased on CD ROM from the relevant states).
These online records are typically the indexes of BDM records considered to be historical - typically births to 1905, marriages and deaths to later dates.
South Australia, Tasmania, Northern Territory and Australian Capital Territory do not have online indexes available. For South Australia and Tasmania the only options are to purchase their respective historical CD ROMs.
NOTE: The Northern Territory only started keeping its own records in 1870, prior to that it was a part of South Australia. The Australian Capital Territory only started keeping their own BDM records in the 1990's. Prior to that they were a part of NSW.
For a a list of what you can (potentially) find on an Australian certificate, the following link is very useful: www.jaunay.com/bdm.html