Importation is a two step process here. The car has to be eligible for import (regardless of LHD or RHD) first, and then once imported, needs to comply with Australian Design Rules and state registration requirements before you can register it.
So in your case, they can be imported if they were made in or before 1988 or if you've owned them for over a year (as a personal import).
However, cars need to be 30 years old before the LHD exemption kicks in (ADR/registration requirement, not importation requirement). I'm not 100% certain on the requirements of campervans for registration, but for cars that's certainly the rule.