Swimming pools are regulated in NSW through a number of Sate Government Acts.  These include the Swimming Pools Act 1992, Swimming Pools regulation 2008

The Swimming Pools Act 1992 defines a swimming pool as:
“.. an excavation, structure or vessel:
(a) that is capable of being filled with water to a depth of 300 millimetres or more, and
(b) that is solely or principally used, or that is designed, manufactured or adapted to be solely or principally used, for the purpose of swimming, wading, paddling or any other human aquatic activity,”

The other regulations primarily deal with fencing and safety requirements for a residential swimming pool.
When Council assesses a swimming pool development proposal it considers matters such as fencing, noise and screening as well as the appropriateness of the proposed location.
How a resident uses a pool is not in question unless it contradicts zoning regulations.

A swimming pool, converted into a pond, that complies with all the fencing, WH&S and site specific development conditions, required by the original development application, does not need further
approval from Council.

The Swimming Pools Act 1992  is not concerned whether your pool has plants or fish in it, whether you chlorinate it etc.  As long as your swimming pool was designed, manufactured or adapted for human aquatic activity, it is still 100% legal and covered by your original DA.

Some local government Environment Health Officers opposed to this program claim that converting a pool into a pond is a new use of the pool, therefore requiring a new DA.  When this opinion has been challenged, to date they have been unwilling to take the matter to court.  WildThings believes that such an opinion is not supported by the Swimming Pools Act which only requires your pool to be designed for human aquatic activity and does not specify particular uses or maintenance regimes.