Motorists face lengthy delays after an unexplained "incident" forced the closure of soutbound lanes of Auckland's Harbour Bridge.

Lane 1 and 2, of 4, southbound were closed for more than 15 minutes soon after 1pm. Motorists were advised to use the middle two lanes - and to expect delays, which remain an issue.

The lanes were reopened just before 1.20pm.

Police and emergency services at the scene asked motorists to avoid the area if possible.

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NZ Transport Agency said there are delays are back to Esmonde Rd due to the incident.