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The old tin roof. Unlikely to be a leak
It is not unusual for moisture to condense on the underside of iron roofs. This is where the dewpoint occurs. On a cold day, warm moisture within the home condenses on underside of the cold roofing material (particularly evident when there is no vapour barrier beneath the floor, no insulation in ceiling and poor ventilation).
On a warm sunny Dunedin day, (common occurrence I am told) you may also have condensation as the roof heats up and moisture from cold damp house condenses on the underside of the roof.

You need to get moisture out of house – vapour barrier (as above) , ventilate bathroom (suspect extractor fan in bathroom may only be ducted into ceiling cavity, which is exacerbating the problem), range hood in kitchen must be ventilated to outside. Obviously insulation will make a big difference in keeping warm air contained within the house.