post 1978 I believe or was it earlier...
Most of the Japanese manufacturers started fitting ADR Compliance Plates to bike sold in the Australian market during 1976. To my knowledge, an ADR Compliance Plate was required for registration from October 1977 in most states.
In some cases you could still get bike registered without a compliance plate after that dated. I was able to get my 1978 Husky 390 AMX registered in NSW in June of 1978 without a Compliance Plate. I was living in Broken Hill at the time, and was told by the official at the rego office that since they had no records for Husqvarna motorcycles locally, that they would register the bike, however once the paper work had been through Sydney, that they may cancel the registration. I never received a notice of cancellation and kept the bike registered until 1981.
A few years ago I had a go at registering my old Husky here in SA. As it was not fitted with a compliance plate, and I stated that it was a 1978 model (therefore made after compliance plates become mandatory), I was told by the local rego office that I would need to take the bike to Regency Park (the SA motor registration head office in Adelaide) and have a full compliance inspection carried out. Unfortunately I was taken ill and had my drivers licence suspended for quite some time, so was unable to make the trip down to Adelaide with the bike for the inspection within the 12 month period that I was given to get the bike passed for rego. I may still have another go at registering it, but will say that it is a 1977 model, and hopefully be able to get it through with out a compliance plate.