However, a trip north to the Central Coast turned to tragedy on Sunday when the family's car crashed into a tree, killing all but one member of the family from Ryde in Sydney's north-west.

Darren and Susana Milne and their 11-year-old son died, leaving their seven-year-old boy on life support in a critical condition.

Police have contacted Mr Milne's aunt in Lake Macquarie, but say they are having difficulty in tracking down next of kin who might take care of the seven-year-old boy if he recovers.

Police believe Mr Milne, 42, may have turned around to speak to his children, causing the silver Toyota Corolla to crash on a straight stretch of Enterprise Drive, between Wyong and Lisarow, about 12.20pm.

Inspector Colin Lott, of the Tuggerah Lakes Local Area Command, said the car hit the tree with such force that the trunk tore through the engine and lodged where the driver's seat should have been.

He said witnesses told police the car was travelling between 90 and 100km/h in the 90km/h zone and did not appear to brake as it veered off the road.

"It hit the tree at full speed," Inspector Lott said.

He said the most likely cause of the crash was inattention.

"It was not a savage turn and the car was only a few metres off the road," he said.

"Most likely the driver turned to speak to the children."

However, Tuggerah Lakes crime manager Detective Inspector Dave Waddell said on Monday that police were investigating all possibilities, including that Mr Milne failed to brake intentionally.

Mr Milne was dead when emergency services arrived and his wife, 39, died shortly after. The two boys were pulled from the back seat.

The older child died of cardiac arrest in the ambulance, while the younger child was flown to The Children's Hospital at Westmead.

The Milnes loved the water and often headed to beaches and lakes around Sydney as well as to the coastal suburb of Redhead in Lake Macquarie to visit Mr Milne's aunt.

Mr Milne was a novice sailor. His wife was born in Mexico and has siblings around the world.

Earlier reports said the 11 year-old boy was the sole survivor but police corrected the information on Monday.

A report will be prepared for the coroner.

- with The Newcastle Herald