A mainframe failure in Australia prevented Westpac New Zealand from making international settlements for most of last Monday.

Westpac Australia is understood to have been upgrading microcode on the mainframe which crashed early on Monday morning. It took out the gateway for the New Zealand subsidiary to access the Swift international settlement network. It wasn't until 10pm that evening that Westpac New Zealand was able to resume making settlements.

A spokesman for the bank, Ian Bonnar, says some customers were inconvenienced but it didn't cost them anything. "There was no material affect," he says. "We put through settlements late at night and first thing in the morning.

"There is a gentleman's agreement among the major banks which allows for some tolerance," Bonnar says.

A Reserve Bank spoksesman says the bank, which had a dispute with Westpac over moving mainframe processing to Australia earlier this year, has no comment on Monday's incident.