Westpac is to launch a review into why its online banking system and card payments broke down.
Customers discovered they could not pay by Eftpos or transfer money electronically during the outage, which lasted more than 10 hours. It also affected mobile banking platforms.
In a social media post early this morning, it apologised for the disruption and stress and said all systems are back online and operational.
Some New Zealand Superannuation payments and beneficiary payments that were due to go out on Monday night were delayed by the issues, but should now be in people's accounts, the bank said this morning.
The delays also extended to some customers of other banks, whose banks receive the government payments via Westpac.
Westpac NZ general manager commercial, corporate and institutional banking Simon Power said staff worked through the night to fix the problem.
"We know this is already a stressful time for many New Zealanders and we apologise for the inconvenience this has caused," he said in a statement.