To the best of my knowledge left hand drive cars didn't actually have to be converted to rhd when imported into Australia in period, its just that many owners chose to convert them to make driving them on the roads easier.
Steve, until (relatively) recently, all LHD cars imported to Australia had to be converted to RHD for road use, but only in certain States and Territories. I think at least Western Australia and the Northern Territory allowed LHD cars to be registered. The Eastern states were not as relaxed!
From around 1999 (at least in my home state), the laws were relaxed to allow vehicles 30 years old, or more, to remain LHD and still be eligible for registration. Hence the steady stream of personal imports of 60's and 70's cars into the country since then, as the previously-prohibitive cost of conversions could be avoided.
However, as far as I know, cars specifically imported for race purposes could remain in LHD configuration, unless they were also intended to be road-registered.