15 year old compliance is different from state to state.
My Ceffy is registered 100% legitimately in VIC with no seatbelts having to be changed (front or back). In NSW where my dad is, its required to have the seatbelts changed to ADR approved ones (aussie A32 maxima seatbelts are the easiest to source). I really couldn't be f**ked jumping through the hoops of NSW's arcane registration system, so the ceffy's remained with VIC rego, even though it lives in NSW for most of the year.
So Mr Eps, regardless of what goes down in WA, it doesn't apply in any other state. All signatories in each state are instructed to follow the state vehicle standards guideline. unfortunately this guideline varies from state to state. You can negotiate all you want, but you can't change the state law... not unless you go thru a long political process or court process anyways.
SEVS on the other hand (non 15yr old imports) are a national set of rules and laws so all states must follow the same rules.