Traffic remains heavy on the M1 Pacific Motorway but all lanes have reopened after an accident on Thursday afternoon.

The Transport Management Centre said northbound traffic was queued more than seven kilometres between Mount White and the Hawkesbury River Bridge.

The accident, which involved a van, closed all northbound lanes for a period just before 3pm.

It happened as Sydney residents prepare to leave the city for the Anzac Day long weekend and join traffic that is already being described as very heavy.