At an Auckland factory this week, owner Norman Kerr removed a rat weighing about 900g - the biggest he had seen and the size of a small kitten.
The average rat weighs 500 to 600g.
"We've caught a few big rats lately and it's all about the available food they're getting to," he said.
"They've been able to access more food outside, but also inside.
"We had a factory stocking noodles and there was the dry type and the type fried in fat. We found the rats were leaving the dry noodles but going for the ones fried in fat - those rats were really big."
The company does a lot of commercial work and is contracted to factories, parks, reserves and other properties in Auckland, Wellington and Christchurch.
Target Pest service manager Tim Brown, who sets up traps and bait in parks and reserves, said he was having to lay down more bait than usual.
"There's a big influx of activity. I don't know if it's overpopulated but we're seeing a lot of the bait go very quickly.
"With the rain now, rodents will be heading inside so people need to be making sure all food is put away properly and checking for holes under the sink and hot water cupboard."
The company was also getting reports of rats being more brazen.
Mr Kerr said: "We've been called out to a few parks where people have reported rats just sitting there out in the open or running up and down trees eating acorns."
"That's not normal for them to be out during the day and for them to be doing that they must be overpopulated and desperate for food."
Auckland Council biosecurity manager Jack Craw said the council was aware of the issue, following a high number of calls from the public, and was making necessary moves.
"The ccouncil has received the expected seasonal increase in complaints and requests for rodent control," he said.
"Our rodent control programme for parks is managed according to the season - for example, the high season is September to April."
Bug King owner Mark McDonald said his company was receiving an increasing number of residential calls about mice.
"No big rats for us. But the real increase we've seen is in mice - calls have increased significantly," Mr McDonald said.
A spokesman for Mitre 10 said figures showed there was about a 20 per cent increase in sales of traps compared to the same time last year.
To report a pest problem on public land, call the Auckland Council on (09) 301-0101.
Getting rid of rodents
Traps
There is a range available commercially to kill rodents humanely. YouTube also has instructional videos on home-made traps such as a revolving can smeared with food bait and strung on wire across the top of a bucket part-filled with water, to which an access ramp is provided.
Poison
Range available commercially.
Electronic rat zapper
Battery-operated to kill rats with electric shocks. Cost: from $85
Humane electronic rodent repeller
Plugs into power points to drive rodents including rats and cockroaches outdoors with electromagnetic, ultrasonic and ionic pulses that irritate their nervous systems. Supplier says will not affect pets or birds. Cost: $150.
Cats
Tend to be unreliable rodent catchers, and may take fright at or ignore larger rats.
