According too the HONDA Motorcycle Identification Guide, 1959 - 1998, (American edition) the first model SL350 (310 model code, electric start engine) had a release date of 15/08/1969 and was on sale during 1969 - 1970. This is not definite proof that the model was available here in Aust prior tp 1970, but if you really want to run a SL350 in the pre 70 class it is worth contacting Honda Australia for confirmation, or putting a request in to MA with the model guide as evidence.
CJ