I would take it as an inertia reel/retractor seat belt, that must remain tensioned and lock upon braking.
Securon in the UK makes a harness style retractable as does Corbeau but they are over the shoulder rear seat mount for the retractor.
Australian DoT compliance is likely different than US, I'd start with some of the aftermarket Australian specific providers e.g.
www.seatbeltsolutions.com.au or seatsafe.com.au