A mass exodus of holiday makers leaving Sydney has led to huge traffic jams along the Pacific Highway.

A traffic jam stretching for about 25 kilometres on the New South Wales mid north coast has added about two hours of additional travel time to the drive north.

Dave Wright from the Traffic Management Centre says delays are at their worst just south of Kempsey.

"[There are] queues around the 25 kilometre mark approaching south Kempsey, which is equating to around 2.5 hours' travel time for those who are actually travelling on the Pacific Highway via that particular location," he said.

"It's all a case of everyone heading north at the same time and as a result we're seeing those major queues today."

Motorists heading to the far north of New South Wales or across the Queensland border are being advised to use the New England Highway to avoid the jam.

There are also lengthy queues and heavy traffic at Woolgoolga, Bulahdelah and on the F3 freeway linking Sydney and Newcastle.

Mr Wright says heavy traffic is expected to continue until at least December 27.

"We're trying to get motorists to ensure that they prepare themselves for a long journey," he said.

"That includes taking bottles of water with them, and just making sure that their car is up to the actual challenge of making a big long journey and the potential of being stuck in a queue for quite some time."