OUSA student support manager Sage Burke said about 75% of students said they had mould in their flats and the issue was so common OUSA had developed a mould-killer solution it distributed to students.
The solution was 70% white vinegar and 30% water.
OUSA could also test the air in a flat for humidity, but students should not be having to do that, he said.
''To rent a property it's a very large amount of money. If you are paying $20,000 a year for something, you expect it to be top quality,'' he said.
The association had received 329 requests for advice on tenancy issues this year and the number of requests for advice increased every year, Mr Burke said.
''The amount of cold and damp and mouldy rental properties that we see is absolutely phenomenal and it's absolutely unacceptable.''
Repairs not being carried out was a major problem and a lack of basic maintenance could cause mould to develop or make it worse.
The worst flats Mr Burke had seen included one with a floor made of shipping pallets, and another with a 2cm gap at the top of the window. He had also heard stories of a flat so humid ''clouds formed inside''.
There were some ''very good'' landlords in Dunedin. However, there were bad landlords who owned several properties and whose tenants might all complain during the year, he said.
Otago Property Investors' Association president Cliff Seque said generally, as long as a flat was well maintained, mould was due to the lifestyle of the tenants, including such things as drying clothes inside, not opening curtains and not airing out their homes.
''You don't see mould in empty houses,'' he said.
However, if there were unresolved problems such as leaking pipes or homes not being weathertight, they could cause mould to grow.
It was not necessary to hire specialists to get rid of mould, and he agreed the problem could be fixed with a mixture of 70% vinegar, 30% hot water and some ''elbow grease''.
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