I have raised this with Fair Trading and got this initial response form my “insider' there:
This stems from the lot air space concept. You buy an “air space” with it's 6 boundaries – walls, bottom and top- and everything outside that is CP. So, re downlights, the technical answer depends on where the airspace ends. If the downlights have glass base, the airspace ends at their under surface, if they have no glass base adn are open to the bulb, then the lot runs up and around the bulb.
However, an OC can decide at meeting not to change bulbs, or, while tedious, they could pass special by-law making all owners responsbile for all ceiling lights within their unit Don't ask me about air conditioning. the open vents have air space runnning up into the ceiling space.
It's one of these issues where strict interpretation of the law bumps up against practical realities and more inquiries are being made.
However, any OC that is seriously concerned that they are going to spend all their time and half their sinking fund replacing blown bulbs in recessed downlights probably needs to get a by-law in place pronto.