I don't know about the 1989, but there used to be a road registered 1974 MX250 in the Penrith/St Mary's, NSW area. I don't think they needed ADR back then. Good luck.
Probably used a dt frame with the mx gear on it
From someone who was riding enduros in the mid 1970s, I can confirm that there were quite a few road-registered MX bikes in use around the Brisbane area. They were mainly used for competition. Back then, many of the big events required road rego. The three Annand and Thompson sponsored riders Peter Payne, Steve Cotterell and Geoff Petler rode road-registered YZs until the ITs came out, and when the ITs first came out, they were only slightly more road compliant than a YZ but no battery or indicators and no compliance plate. A few years later on when the Main Roads dept started getting serious about motorbikes needing ADR compliance plates, the Yamaha distributor would post out your compliance plate for your new bike when they were made.
Fond memories of checking out a road registered YZ125C being prepared at Annand and Thompson
Another riding friend had a road-registered CR390 Husky which I think was a 1978 or 79 model
Later on (late 1980s) I lived up North and found that it was commonplace for people to be racing current model YZ125, YZ250, KX250 and CR125 bikes in enduros. One bloke I rode with worked at Cairns Yamaha and he had one of the first Yamaha YZ250WR (I'm thinking it must have been about 1988 or 1989) and it was a pretty close thing to a normal YZ250, and it came with a compliance plate (fairly amazing to have a compliance plate on that thing I thought at the time)