I believe the last Polar bears in Australia (prior to Sea World’s imports) were the pair at Adelaide Zoo (Bjurn and Tasha). They were sent to Taman Safari in 1992.
Taronga Zoo’s last Polar bear was a female named Ingrid. She was sent to Auckland Zoo in 1984 after the death of her male companion.
I can find an article about the death of a male Polar bear at Melbourne Zoo in 1980. An article from 1982 details complaints over the bear’s living conditions - proving they still held the species.
If you’re wanting to know the last death (not the last Polar bear), I’d imagine it occurred at Melbourne Zoo around the mid 1980’s; or possibly Adelaide Zoo around the same time. The pair they exported to Indonesia were from a group of four imported in 1976 - with the other two dying sometime before 1992.
The last Polar bears in New Zealand died October 1995 and December 1995 respectively.