well i take back what i said, its only mandatory for vehicles manufactured after oct '91-
http://www.comlaw.gov.au/Details/F2009C00579
so what is the manufacture month date on your car given it is a '91?
if it is pre legistlation you could always just print out the details of the link i posted and argue the point with the police officer who pulls you over as they have a limited understanding of the ADRs at best, or you could just install the light! seriously its not that hard a job. Anyway it may actually prevent some numb-nut running up your arse one day.
btw the mirrors are pretty much the same deal. think i got confused as i remember when i was a kid, mum got defected for a blown tail light and so dad installed a second mirror and a high mount light at the same time and then got it cleared at the local cop shop. this was in a '79 Mazda 323 and would have happend in the late '80s/early '90s. they started phasing those laws in from '88 so grifty was right. guess dad was just being proactive, either that or the cop gave him a bum steer!