Is it possible to use LHD wiring from a 70 to 73 usdm model in a 1980 Rhd aussie spec 1200?

sure it's possible just need to make some mods as the newer connectors differ

late vs early there are some differences in front combination lamp wiring and in rear combination lamp wiring

Also the dash harness is reversed

Probably not without some work, the instruments are on different sides. The wiring will probably need to be reoriented to be able to hook up the gauges and switches. The wiring goes through the firewall at the right front corner of the engine compartment on LHD models and seems to do the same on RHD models, that would give you about 2 feet (60 cm.) difference in where things hook up. Also the plastic wire holders at the harness connectors are probably different too.