"This error was made in 1989, so it goes to show that no stone is too small or too far away to remain unturned," he said.
The mistake was spotted by eagle-eyed councillor Stuart Bell.
"I take my hat off to councillor [Stuart] Bell for bringing it to our attention, and to staff for taking action very smartly," Mr Forlong said.
He said legal advice sought by WDC had revealed "that this has no effect on the legalities of any of the decisions made by councillors sworn in under the old wording". It did not impact the legality of previous councils, either.
But, WDC would "tidy it up right away with no messing about. It means that each declaration for this term and for future terms will be correct," Mr Forlong said.
A Local Government New Zealand spokesman said LGNZ advised councils to ensure they were using the right wording in such declarations.
"Councils should ensure they are using the correct declaration as required by the act - the LGNZ handbook for elected members provides clear guidance about the declaration and the correct wording."