ASB's KiwiSaver customers have been hit by an online outage which stopped them from seeing their account balance.

The bank is the second largest KiwiSaver provider and had $8.9 billion invested via its scheme as of June 30, according to figures from Morningstar.

Normally people can log into their internet banking and see their KiwiSaver account balance next to their other accounts.

But several people contacted the Herald and another posted a comment to the bank's Facebook page to say they could not see their KiwiSaver balance online this morning.

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An ASB spokeswoman said it was aware of the problem but it had since been fixed.

"We are aware that some customers had difficulty viewing their KiwiSaver balances this morning, our team has since resolved this issue.

"We appreciate our customers' patience and we apologise to anyone who was impacted by this."

The bank did not say what had caused the glitch or how many people were affected by it.