David McPhee Pest Management Services owner David McPhee, of Dunedin, said the hot summer was ideal for rats to breed and the cold winter had an ''influx of rodents'' seeking shelter indoors.
He had been working seven days a week to keep up with demand.
''We are extremely busy. We are the busiest we've been for 30 years.''
He recently trapped 27 rats in a commercial Dunedin building, he said.
But he used poison too, so a precise head count was difficult to determine.
Rats were as cunning as ever, he said.
''We put an infrared camera in a roof and watched them taking bait from a trap and not setting the trap off.''
Procare owner Paul Corkin, of Dunedin, said he had been exterminating rats for nearly 20 years and his pest-control business was busier than usual.
Rats were infesting entire houses, rather than living in one area of the house, such as the kitchen, which revealed a greater rat population than in past winters, he said.
Rats in a Central Otago farm house had eaten 20kg of bait in a week, he said.
The rodents then ate another 15kg the following week and 10kg the week after, he said.
''They had quite a problem, when you consider that 20g kills a rat.''
However, an alpha male rat would eat more than the average rat, he said.
Target Pest South Island director Kurt Loklindt said he had exterminators working across Otago.
The rats were the same size as in past years but if a rat lived in freezer panels, it could appear to be larger, because the cold made its hair grow longer.
It was unusual to find a rat that weighed more than 1kg, he said.
A house on a rural block was three times more likely to be infested with rats than an urban house because in the country, a rat had better environment in which to breed, he said.
