I'm not certain although given the proximity of the water reservoir nearby and what the structure looks like in Google Maps, my guess is that it is a Council water booster pump station associated with the reservoir or is housing other infrastructure associated within the water reservoir.
Source: I'm an Australian engineer and work on similar structures. It could potentially be a small sewerage pump station although the location, the lack of obvious overflow storage and the presence of the water reservoir leads me to believe it would be associated with the Council water system.
Eitherway, in a lot of Council's in Australia, much of the infrastructure sites and locks all use a common key (e.g. a literal 'key to the city' etc) and which dozens if not hundreds of people would usually have access too or a copy if anything nefarious happened in this case in the proximity of the building. Sites like these are usually remote monitored (via infrastructure telemetry/reporting sent via phone/text/internet, not CCTV) although would often be visited by Council operational staff at least once a week routinely or so or when something breaks or is unusual.
