Contractors trying to stop the spread of avian botulism in North Canterbury now estimate that 1000 birds have been killed.
Dead birds were discovered in oxidation ponds in Kaiapoi earlier in January and contractors are now removing carcasses to contain the outbreak.
Waimakariri District Council spokesperson Gerard Cleary said on Wednesday that the ponds are monitored several times a year in the Brooklands Lagoon area and the previous check on 6 January showed no evidence of the disease.
Mr Cleary said by Tuesday, there were about 1000 dead birds on pond banks and in the water, with a further 20 showing classic symptoms of avian botulism - lethargy and partial paralysis of the feet and wings.
The disease, which cannot be contracted by humans, is relatively common with several outbreaks in the greater Christchurch area in the past few years. However, this is the largest known outbreak at the Kaiapoi oxidation ponds, the council says.
The species most affected to date are: Paradise Shelduck, Black Swan, Mallard, Grey Teal and New Zealand Shoveler. The Kaiapoi ponds typically contain a total bird population of between 5000 and 6000 birds. The majority of these are currently moulting, which renders them flightless for a short period and thereby increases the risk of them contracting the disease.
Minimisation of the outbreaks consists of the removal and safe disposal of dead carcasses, which prevents the spread of the disease. The outbreak is unlikely to end completely until the arrival of cooler weather, with frosts breaking the re-generation cycle of the disease.