A botulism outbreak at an internationally significant wetland at Miranda in the Firth of Thames has killed hundreds of game and migratory birds including some endangered species.

Fish & Game, DOC and the Miranda Shorebird Centre have been working to remove the dead birds since the weekend.

Waders at Miranda. Photo: DOC

Avian botulism outbreaks are often sparked by warm temperatures and water with low oxygen content. Botulism can paralyse and kill birds.

Miranda is recognised by the international Ramsar Convention as a significant global wetland worthy of protection and conservation. It is home to thousands of waterfowl and shorebirds, many of which are rare or endangered. It is also a stop off point for migrating godwits.

David Klee, Fish & Game's southern game bird manager for the Auckland/Waikato region, says the situation is grim, with the botulism taking a toll on a wide range of species.

'Some of the species we are finding dead are endangered and they have died slow and agonising deaths,” says David. 'All we can do is pick up and dispose of the dead birds but the extent of the outbreak is upsetting for our staff and volunteers and staff from the Miranda Trust.

'The Firth of Thames is prime habitat for waterfowl, shore and sea birds and should be regarded as a treasure, something people can visit and enjoy. Botulism is symptomatic of severely degraded environments and sadly, the area is becoming increasingly polluted and it is time government and local authorities took a tough stand.

'Polluters responsible for this situation are not going to do anything for the good of the environment and the public unless they are forced to.

'Some industries are using the surrounding rivers that enter the Firth as a dumping ground and seem to believe that it is their right to continue to discharge increasing volumes of contaminants.”

If the Firth collapses, it won't just be the bird life that will suffer, says David. There is the potential for the multi-million dollar aquaculture industry and huge recreational fishery to also be adversely affected.

The botulism outbreak is not the first indication of the Firth of Thames' deteriorating condition.

In 2014, NIWA research scientist Dr John Zeldis warned that water pollution from decades of farming may mean the Firth and the wider Hauraki Gulf is reaching a "tipping point" which threatens marine life.

Dr Zeldis' research showed that water in the Firth became low in oxygen in late summer and autumn and had become increasingly acidic because of a build-up of nutrients from rivers contaminated by run-off from Hauraki Plains farms.

David Klee fears that unless there is a major effort to clean up pollution, the Firth can only deteriorate further.

'The Firth of Thames is getting worse. There needs to be urgent action to start cleaning it up otherwise there will be more botulism outbreaks and they will get worse, putting the status of the Miranda foreshore at risk of being an internationally valued Ramsar wetland,” he says.