The general rule
The party whose parcel or property caused the leak is responsible for repairing the defect and for making good the damage it caused below. If the source is inside the unit above (a failed bathroom waterproofing membrane, a cracked pipe under the tiles, a leaking floor trap), the owner of that unit pays. If the source is common property (a riser pipe in the duct, a corridor slab, a roof), the management body pays from the maintenance fund.
The Regulations also contain a presumption: an inter-floor leak is presumed to come from the parcel above unless it is shown otherwise. That is why the inspection matters so much. Without it, the upstairs owner carries the presumption; with it, everyone knows the real cause.
Who decides
Not the contractor and not the neighbour. The management body (the JMB or MC) inspects both units, decides the cause and the responsible party, and records it on Form 28, the Certificate of Inspection. Form 28 is the document that turns "I think it is from upstairs" into a written finding the responsible owner is obliged to act on.
Our role is to carry out the inspection and the tests the management relies on, and to repair the leak if we are appointed. We do not decide responsibility; the certificate does.
What if the upstairs owner refuses
Form 28 gives the responsible party a fixed period to rectify. If they do not, the management body may carry out the repair itself and recover the cost from that owner, and the matter can be taken to the Strata Management Tribunal. In practice most owners act once they receive a proper certificate with an inspection report and photographs attached, because the evidence is clear.
What you should do first
Photograph the ceiling and note when the water appears (after rain, after the neighbour showers, all the time). Report it to your management in writing so the notice date is recorded. Then report it to us; we inspect both units, run the tests, and issue the written report your management needs to prepare Form 28.
Report it with photos and you get a case number immediately. We inspect both units and issue the written report your management needs.