A ''rat tsunami'' is infesting homes and threatening wildlife across Northland as a booming rodent population munches its way through anything in its path.
While there's nothing unusual about rats seeking warmth in people's homes during winter, pest control firms say this year's rodent invasion started earlier and in numbers far higher than normal.
The population explosion is irritating, and frightening, for home owners and is a cost and hassle for hospitality businesses. For native wildlife, it can be disastrous, as the rat plagues are linked to local extinctions of birds, bats and insects.
The cause is thought to be a ''mega-mast'', when native trees and shrubs produce unusually large amounts of fruit and seeds. Plentiful food and mild temperatures allow rats to breed quickly. Once the seeds have been devoured, the voracious rodents turn to native wildlife as well as bins and pantries.
• Brazen cat-sized rodents causing headaches for Titirangi residents
• Titirangi residents share hilarious memes about 'cat-sized' rats on local Facebook page
• Auckland Council helps pay for backyard rat trapping credited with bringing back native birds
• It's not just Titirangi: Rat infestation doubles across New Zealand
Brad Windust, of conservation group Bay Bush Action, said volunteers caught their 10,000th rat last week, despite only trapping since 2011 in a relatively small area of bush behind Paihia.
''There's always a seasonal bump in catches this time of year but right now we're trapping what feels like a tsunami coming in. It's a double whammy as rat plagues like this also dramatically increase stoat numbers.''
Windust said he feared a repeat of previous localised extinctions.
''Northland has already lost far too much to the mouths of these introduced invasive predators. Our forest parrots the kākā and kākāriki are gone, along with bellbirds and saddleback,'' he said.
The problem is not confined to the Bay of Islands or to native bush. Pest control companies around Northland and the rest of the country say they are fielding many more calls about rodents than usual.
Donations flood in for environmental group hit by trap thefts
Northland man's pest trap targets different species
John Clarke, of Waimate North-based Pestworx, said numbers were at least twice as high as last year.
''They also seem to be a lot more brazen this year, because there's so many of them I guess. We're getting lots and lots of calls and going through buckets and buckets of bait.''
Brandon Smith, of Northpest in Whangārei, started fielding calls about rats in March - about three months earlier than usual.
Calls for advice and pest control were up at least 50 per cent on last year. House mice, black rats and Norway rats are the main problems.
''It's going to take a bit to get them under control,'' he said.
Forest and Bird Northland conservation advocate Dean Baigent-Mercer said pest control groups around New Zealand were reporting high rat numbers, from the tree tops to the forest floor, in urban areas and on beaches.
''They're everywhere,'' he said.
''As weather turns cold and food from native trees dries up, the rats are looking for other things to eat so they will head into people's homes and eat native birds, bats and wētā. They will be eating their way through the ecosystem.''
Given the right conditions, a rat could have 10 babies every two months, he said.
Masting, or producing more seeds than usual, was common to all plants but this year a large number of species — such as flax, cabbage tree and kahikatea in Northland — had masted at the same time in a ''mega-mast''. Before humans brought pests to New Zealand, mast years were times of abundance when native species ate and bred well.
''But the years that used to benefit native birds and bugs are now years of terror, population collapse and extinction,'' Baigent-Mercer said.
Because half of stoats' diet consisted of rats, a rat boom was always followed by a stoat boom, which had serious consequences for kiwi populations.
Rat infestations are being reported around the country. Titirangi is also grappling with giant-sized rodents. Residents of the West Auckland suburb have reported seeing rats the size of small possums.
How to keep rats out of your house
• Try to rodent-proof your home by blocking holes and sealing gaps around pipes. Block any gaps under doors.
• Trim trees or bushes overhanging your roof. A rat can easily jump gaps a metre high or wide.
• Keep anything rats might eat in closed containers.
• Store rubbish or food scraps in metal or thick plastic bins with lids.
• Don't leave rubbish bags outside overnight.
• Don't leave extra pet food out.
• Clear weeds or ''harbourage areas'' around your property and keep an eye on compost bins.
• Use poison baits or traps around the outside of your house first to stop rodents getting in. Use bait inside as a last resort.
Tips from the Ministry of Health, Pestworx and Northpest.