Several invasive mangrove seedlings have been deliberately planted in the Ahuriri Estuary, the Department of Conservation says.
DoC said it was concerned by the potentially "catastrophic" discovery, made by local biodiversity champions.
Ahuriri Estuary is an important ecosystem in the region and provides a habitat for endangered species like the kuaka (eastern bar-tailed godwit), spoonbills, dotterels and occasionally terns and herons.
DoC biodiversity senior ranger Denise Fastier said the introduction of mangroves could devastate that habitat.
"The reckless introduction of these plants to Ahuriri Estuary could be catastrophic for the delicate estuarine ecosystem that supports so many threatened species of wading birds."
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The mangrove seedlings were discovered recently on separate occasions by locals Mike Hockey and Hans Rook, who removed them and immediately contacted DoC.
In some areas mangroves can offer some environmental positives, but in Ahuriri they have not historically been a part of the environment and if they were to establish, Fastier said, they would choke the estuary and upset the balance irrevocably.
Anyone found planting mangrove seedlings in Ahuriri Estuary could find themselves facing prosecution under the Conservation Act, 1987.
Hawke's Bay Regional Council spokeswoman Anna Madarasz-Smith said mangroves are only naturalised in areas north of East Cape.
"They have many great qualities including binding sediments to reduce erosion, habitat for fish and invertebrates and filtering water to improve water quality.
"However, they can also grow in excess where sediment coming in from the catchment is changing estuarine habitats, and some areas are having to remove them," she said.
"Hawke's Bay is not within mangroves' natural range and we advise strongly against trying to introduce them to estuaries in this area."
Even where mangroves are part of the environment, density has rapidly expanded to the point of congestion.
Northland local authorities have responded with mangrove management projects.
These focus on restoring coastal access, firm sand flats, shellfish beds and improving biodiversity by removing mangroves.
If you see mangroves in Ahuriri Estuary, contact DOCHOT on 0800 362 468.