There are only between 150 and 200 of the birds left in New Zealand.
Attempts to catch the bird and remove the ring have so far proved unsuccessful.
Avon-Heathcote Estuary Ihutai Trust spokesperson Tanya Jenkins was furious at what had happened to the heron.
“Many of us will have seen this stunning heron and enjoyed its presence in the urban waterways of the city,” she said.
“How incredibly sad that in return for its trust in us, we have given it a choking collar of urban waste that could threaten its life.”
The trust will hold its ‘Mother of All Clean-Ups’ on 12 May, when hundreds of volunteers will hit the city’s waterways to clear them of rubbish. - RNZ