While it might not yet be a plague, the rat population has exploded thanks to a warm start to the winter and a feast of beech mast seeds, pest controllers say.
And the rodents are squatting in houses to keep warm and away from the wintry conditions.
Rentokil NZ regional technical manager Bill Paynter said there had been a significant number of callouts from residential properties this year.
Many calls had come from clients with beech trees - which provide a favourite food of the pest.
"The other thing we've had is a reasonably warm winter ... and when you get warm weather like that, mortality in the nest is significantly lowered. You will get potentially eight baby rats coming out when normally you'd only get two."
In Auckland, callouts to eradicate rats from homes had increased by 33 per cent on last year, Mr Paynter said.
"I think potentially some of the active ingredients in our rodenticides have been around for quite a few years now and we might well be getting some resistance to those."
About every six weeks, mature rats had a litter of six to eight babies, he said.
"And then they mature in three months - so you've got the potential for quite an explosion of rodents."
Houses near trees, where branches were rubbing against the building, were in danger of rats crawling across the branch and into the roof cavity, Mr Paynter said.
Areas that had suffered the worst infestations included Northland, Auckland and the West Coast.
Target Pest New Zealand South Island director Kurt Loklindt said their workload would normally slow down by this time of year, "but we're still getting lots of callouts".
Plants were starting to grow and flower earlier than usual. This, combined with extreme weather such as flooding, had probably contributed to the increase, he said.
Department of Conservation senior services ranger for Whangarei Nigel Miller said the best way to kill rats was with traps and poison.
"Kill them away from your house," he said.
"[Set] two or three traps around your house, up to 100m away. Get on to them before they get into your house."
Getting rid of rats:
• Young rats can get through holes of less than 1cm, so ensure all holes are sealed and fit bristle strip around doors.
• Be careful with dog or cat flaps as rats can get through them.
• Make sure all pipe-work is in good order, as rats can also enter properties from the sewer through broken pipes.
• Make sure your house is clean, including under the cooker and fridge, and all household refuse is kept in closed bins.
• Do not put meat scraps in the compost heap, use squirrel-proof bird feeders, and bring in pet food bowls after feeding.
• Poison and rat traps can be used both inside the home and out, and can be easily bought in hardware and DIY stores.
(Source: Department of Conservation and Rentokil)
Additional reporting: Northern Advocate