The back of house areas are quite useful. Melbourne's held an array of species off display over the years whether it's birds, monkeys or even small and big cats. You can't forget the many off display facilities they also have for the smaller endangered species they breed; insects, reptiles and the like. The main ones are behind the Tigers and I believe that's the main off display housing area. The area behind Growing Wild is mainly the 'lorry' area used for deliveries of food and the sort. The area behind the Gorilla Rainforest is then the 'staff' area with cottages for directors ect. and also education buildings. So everything sort of does have a use, Melbourne is Zoos Vic's 'headquarters' after all; so a lot of their senior office staff do work there too.
The Japanese Garden isn't overtly large so I don't have a problem with it. It's a nice little area and can be a cool place to sit down and watch the saimangs when they're on the island.
Historically, the vet clinic was also used to house one of Melbourne’s male orangutans off display for a period of time. This was in the early 1990’s, so unsure if the set up remains unchanged in this time; but would assume they have extensive back of house housing for animals undergoing treatment/observation.
Quarantine facilities would also be needed, which are required to be isolated from areas used by existing individuals. The back of house at the tigers would fulfil this requirement as they could be held away from their resident tiger. They would have last utilised this for quarantining a tiger in 2004, when Binjai was imported from Europe.