Sydney does have GBAS, on an approval-required basis.
If you want to fly to precision approach minima on a GPS derived position fix, the GPS data has to be augmented ("corrected for errors").
You can do this from space (SBAS / WAAS, wide area, expensive to run but cheap to use).
You can do it over a region (can't remember acronym), say 100nm x 100nm, but
AFAIK no-one does.
You can do it for a terminal area (GBAS) with a base station and VHF data link to participating aircraft (small area, cheap to run and moderate expense to use).
In all cases the principle is the same. Laptop bolted to the ground assesses GPS position, compares it to true position. Uplinks error correction to satellite or participating aircraft. If satellite, correction is re-broadcast to participating aircraft.
There are other benefits too, such as real-time re-broadcasting of satellite health and integrity to a better standard than RAIM.