Online shoppers will no longer be able to avoid an undisclosed fee charged on overseas purchases made in Australian dollars.

Westpac has joined the other big four banks in slugging consumers with an ''international transaction fee'' on credit card purchases made with overseas merchants.

The new online fee has caught out some shoppers. Credit:Penny Stephens

The NAB has also fallen in line with the other banks by raising its fee, which was between 2.3 per cent and 2.5 per cent, depending on which type of card was used, to the 3 per cent level charged by its competitors.

Westpac customer Peter Copleston, of Sydney, said he was charged the fee when buying plane tickets from the Air Canada website and again when shopping at Amazon.com.