Sergeant Shaw said that as the boat was capsized by a wave, only the 7-year-old was wearing a life jacket. There were other life jackets on the boat but they were not in use.
Coastguard representative Brian Grimwood said the conditions at the time were moderate to rough.
When the boat capsized, all members of the party were separated but managed to return to the boat.
It is unclear whether the missing man returned to the boat at this point.
The members of the party were then separated a second time, and the man's three children and the 14-year-old friend managed to swim to Matakana Island.
On Matakana Island the four flagged down passing fisherman Warren Coffey and his friend who were returning from a fishing trip.
Sergeant Shaw said the search was continuing for the missing man and this would be reassessed later today.
``We always hold a glimmer of hope but as the search progresses, we have to be realistic with what we are looking for.''