Hundreds of dead birds are suspected to have been killed by an avian botulism outbreak near the Firth of Thames.
Fish and Game Auckland/Waikato southern game bird manager David Klee said they'd been informed on Wednesday about large amounts of dead birds being found alongside a drain near Waitakaruru.
To prevent the outbreak from spreading, Klee put an urgent call out for volunteers in the area and along with several other agencies conducted a mass clean-up operation on Friday, closing the nearby rail trail for the day.
The team of about 30 people collected about 800 dead birds across a 15km stretch, and managed to rescue six that were taken to NZ Bird Rescue in Green Bay.
"We've had some outbreaks in the wider area in the past but not in this drain, and it looks to be a pretty bad one.
"Those were just the birds we could find, but there are likely to have been a lot more killed as the outbreak might have been going for about three to four weeks, and we were just picking up what the scavengers have left."
The majority of the birds were duck species, including mallards and grey ducks, but also included some herons and shags.
Avian botulism was a toxin produced by bacteria naturally present in sediment, but only became active in favourable conditions - hot temperatures - and when it latched on to a "protein substrate" such as dead animals, Klee said.
In summer and particularly during dry periods when streams and creeks ran dry and fish and invertebrates died, the botulism would often become active, and could eventually spread to birds feeding in the area and drinking water.
After a bird died from the toxin, the botulism could then be easily transferred to other birds through maggots.
"That is why it is important to get onto it and remove the dead birds as quickly as possible from the system, to prevent it spreading," Klee said.
"There are some really significant bird populations nearby, with endangered and threatened birds, so we wanted to get onto it as quickly as we could."
Klee said botulism outbreaks were becoming increasingly common with climate change.
"It is not uncommon to find these outbreaks, but it does appear to be getting worse as the climate changes and we start getting longer, hotter summers."
Once there was an outbreak in an area it meant was more susceptible to another in the future.
There were generally no risks to other species with the strain of botulism typically found in New Zealand, but Klee said people should take normal precautions if they saw dead birds and not pick them up without gloves, and keep their pets away from them.
If anybody found a large number of dead birds they should contact Fish and Game, Klee said.
Fish and Game worked alongside the Upper Piako Wetland Management Association (which comprises a group of hunters), the Department of Conservation, Waikato Regional Council, Ngāti Hako, the SPCA, Bird Rescue and the wildlife rehab network and local landowners on the clean-up operation.
The rail trail has since been re-opened.