Why is my Australian light fixture wired this way?
It's fairly standard



In your case the red/white pair is probably from the switch, red probably live and white switched live (normally in my part of the world it would have red tape on it to indicate switched live). The other two twin+earth cables are mains supply and continuation to next light fitting. Your wires were connected directly to the light fitting so there is no separate cable to the lamp.

You can confirm this with vary careful one-handed testing using a suitable Cat II rated mains voltage tester. You can get non-contact voltage testers that make this safer. 

After testing and confirming above, I'd connect the new lamp to the black group (AC neutral) and the white wire (switched live)