An operation to catch a rat on Tiritiri Mātangi Island has cost the Department of Conservation (DoC) $40,500.
Rat tracks were discovered on the pest-free island on January 7, leading to a three-week operation to find and kill the rodent.
DoC Auckland inner islands operations manager Keith Gell said the major cost was people working seven days a week.
- Rat on the loose on pest-free Tiritiri Mātangi Island
- Department of Conseration on the hunt for rat on Tiritiri Mātangi
"It's not just folk on the ground, but also the controlled incident management structure we put in place - the advisory group, the boat crews, the specialist dog team, that sort of thing," he said.
"I'd say a third of it also went to things like traps, bait, tracking cards and transport out to the island."
Mr Gell said it's important to keep an eye out for pests when visiting pest-free islands like Tiritiri Mātangi.
"Boat owners need to make sure there isn't a rat or a mouse stowed away on their vessel, whenever they're heading out to sea in the Hauraki Gulf," he said.
"People staying at campsites on pest-free islands must remember that cats and dogs are not allowed on pest free-islands. And they must not interfere with traps and other biosecurity devices on these islands."
Tiritiri Mātangi is home to a number of endangered native New Zealand species including takahē, tuatara and kiwi.
DoC isn't the only public organisation with a big pest control bill. In the last five years, Auckland Council has spent around $131,000 a year killing rodents in the region.
Biosecurity principal advisor Dr Imogen Bassett said that's because rodents are more abundant in Auckland than other regions in New Zealand.
"[It's] due to our mild climate, extensive coastline, and abundant year-round food sources.
"Rodent populations fluctuate year to year in response to environmental conditions such as weather and food availability.
"For example, when native trees experience a year of fruiting heavily, known as a mast year, this can promote an eruption of rodents in our native ecosystems."
Auckland Council operational spend for managing rodents:
- 2012/2013: $115,000
- 2013/2014: $151,000
- 2014/2015: $131,000
- 2015/2016: $139,000
- 2016/2017: $121,000
Dr Basset said this money is put toward community activity, control and monitoring on regional parks and work on Hauraki Gulf islands.
The figures do not include associated staff costs or work relating to rodent control at local parks.
An additional $600,000 was spent on was spent on the Hunua 1080 operation in 2015. However, this targeted possums and stoats in addition to rodents.