Locals say they watched missing catamaran leave the Kawhia Harbour 10 days ago with plenty of supplies on board.
The six-metre boat was travelling to Paihia with 49-year-old Alan Langdon and his six-year-old daughter, Que, on board.
They left Kawhia on Saturday, December 17 bound for the Bay of Islands but haven't made contact since.
Police have grave concerns for their wellbeing and are seeking information from other boaties who may have seen them over the past few days.
They say Mr Langdon is an experienced a capable skipper and his daughter has had experience on boats.
Waikato Maritime Services leader Richard Barnett says he'll be assisting police where he can.
"Search and Rescue will provide assets on the water or the Police Dive Squad to deploy when they search in our region, so we cooperate when we can, but we're always led by Police Search and Rescue."
Mr Barnett says it's a long trip to the Bay of Islands, especially over the unpredictable West Coast.
Kawhia locals say they saw the boat leave with plenty of supplies on board.
The catamaran is white with blue anti-fouling paint under the waterline, and the sail ID number is T878.
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