Airfields may only be closed from the public domain and yet they may still be actively used by their owners for private operations, ranging from bush pilots to agricultural applications. Other airfields might be classed in the 'emergency use only' category. There is another category, where an airfield has been closed for public (and possibly private use) use but there is a lag in its transition to another land use, so for a period they may well be available for emergency use. How long this transition may take is anyone's guess. A grass airfield could be ploughed and a pivot irrigator installed and it is effectively no longer available as an airfield. Others may take much longer. I have had the opportunity to land on airfields that have been 'transitioning to something else'.
Tracking the transition of a closed or abandoned airfield is difficult as it either requires a network of people to report and/or good access to current satellite imagery with sufficient resolution across large tracts of Australia. Satellite data may lack the metadata showing the date of acquisition of the imagery so it can be very difficult to report on changes.
For example, in Tasmania, there has been a history of airfields constructed on forestry land. Purely speculation, but some may pre-date the more common use of rescue helicopters in the event of accidents near forestry operations. These airfields are often in a 'to be verified' status as there may be remnants of an airstrip but regrowth of plantations may make emergency use very difficult.
Other reasons for the closure of airfields include the development of wind farms. In Tasmania, I know of one at Cape Portland in the north-east. Robbins Island in the north-west has also been identified for a wind farm, which may also result in the closure of this island airfield. Urban development whether for housing or commercial use is also resulting in pressure to close airfields. Increasing in housing densities together with price increases may often combine to create re-zoning pressures. Cambridge north of Hobart will shortly be closing RWY 14/32 to allow for commercial development.
Then there are the airfields which rise again! In some cases, a new enthusiastic owner may bring life back to an old airfield or create a new airfield. Others are more mysterious. A case in point is the Lake Pedder airstrip near Scott's Peak in south-west Tasmania. This has been closed for some decades and has been fully re-vegetated. It disappeared off aeronautical charts only to re-appear again in 2020. The Lake Pedder airfield also has its own ALA Code in ERSA. Can you find the mysterious airfield in the satellite image below?
Finally, I'd like to make a recommendation for greater use of OpenStreetMaps for managing the very difficult task of keeping track of the availability of unregistered/uncertified airfields on public and private land. Crowd-sourcing is probably the only feasible way to maintain some level of documentation of these important assets.