Greenpeace has been found guilty of misleading advertising in a television campaign about oil pollution.
The Advertising Standards Authority says an advertisement saying 20,000 birds were killed by the Rena oil spill was likely to mislead and deceive, and was in breach of advertisers' code of ethics.
Energy analyst Bryan Leyland complained to the authority, saying official figures showed 1300 birds died.
Greenpeace said 2000 were known to have died and that overseas research indicates only one tenth of bird deaths from an oil spill are detected and most die unnoticed.
But the authority said the claim of 20,000 dead birds was presented as a strong and absolute statement of fact.