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.
That is why the inspection matters. Until the management body has inspected both units and issued Form 28, who is responsible is only a guess.
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
A parcel owner named on Form 28 must rectify within 7 days of receiving it. If they do not, the management body may carry out the repair itself and recover the cost from that owner. An owner who disagrees with the finding can refer it to the Commissioner of Buildings (COB), and claims can go to the Strata Management Tribunal (TPS). 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.