Auckland Council says there's been a significant rise in reports of dangerous and unsanitary buildings that are being passed off as rental accommodation.

Auckland Council general manager building control Ian McCormick says the council takes a tough stance on anyone profiting from illegal garage conversions or substandard rentals.

Last year the council issued more than 700 notices requiring owners or landlord to 'fix' their properties - most related to buildings being used as accommodation without consent.

But in just the past six months, the number of complaints in central Auckland alone jumped to 750, with a further 600 in south Auckland - almost 500 of which related to unconsented dwellings.

Mr McCormick says: "In the last couple of days we had a complaint… where someone has divided up a three-bedroom house into three residential units and also divided up the garage into two further residential units."

The dwellings were illegal, and the motive he says was to maximise profit.

Another recent case involved a carport with makeshift walls and an outside kitchen connected to the power from the main house. It was being rented as a separate flat by a real estate agent.

The council's also investigating the illegal conversion of a former piggery into eight apartments.