In Sydney Australia, how do I connect a 2-wire (red, blue) ceiling light to a 3-wire (red, black, green) ceiling power source?
See Australia: Which wire is hot / active and which is neutral? for some more details, but basically:
- Hot: Red -> Red
- Neutral: Blue -> Black
- Ground: Green
Typically, if you have a fixture that does not have a separate ground wire then there will be a screw attachment point on the fixture to connect the ground wire.
Australia's historical color for neutral is black. (Following UK's lead for black=neutral red=hot, a convention still used most places for DC power).
The new 4-continent standard for neutral is light blue.
So it's easy to see by context that your legacy Australian wiring has black as neutral, and your modern world-market lamp has blue as neutral. Join those.
Grounds are worldwide standard as green, yellow/green or bare wire.
That leaves two remaining wires that must be the hots.