G’day Simone,
I haven’t had this situation before, but I think I would be investigating the Limitation Act 1969 section 15. I interpret it to mean you need (only) refund 6 years of overpaid rates.
http://www5.austlii.edu.au/au/legis/nsw/consol_act/la1969133/
Maybe someone else has a definitive answer?
Regards, MO.
