Vodafone customers have been left confused after an outage took out access to the company's app and much of its website.

The Herald has been contacted by a number of customers about the outage, which had been planned to enable an upgrade to systems.

The network is not affected. But Vodafone's call centre and staff in its stores are unable to look up customers' account details.

A spokeswoman said the outage to the app and website was from 8pm last night until this afternoon.

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The work had been publicised by Vodafone including on social media.

Customers with low pre-pay balances who normally top up using the app had also been identified and warned before the outage.

"It has actually been years in the planning ... it's just about upgrading our systems so down the track we can add functionality and make things easier for customers.

"As I understand it, we are still on-track to come back online this afternoon."