The Bank of New Zealand has been fined more than half a million dollars for breaching the Fair Trading Act.

The Bank's also been ordered to pay its customers $5 million dollars in compensation over a failure to adequately disclose currency conversion fees on credit cards.

The BNZ's become the third major bank to plead guilty to breaching the Fair Trading Act, admitting 21 representative charges.

It must also pay costs to the Commerce Commission, which brought the prosecution.

Customers will be contacted by the BNZ if they are affected, and most should receive their compensation by November this year.