The US Coastguard says a report of a yacht - possibly with a Kiwi crewman - being blown up and sunk could be a hoax.

Maritime NZ won't reveal details of how the $37 million Rena clean-up bill has been spent.

The container ship captain responsible for NZ's worst environmental maritime disaster tried to cover up shoddy seamanship by ordering records to be changed - and then kept quiet about it.

A Mangere man, 45, and his 7-year-old son had been missing since their aluminium dinghy tipped over under the old Mangere Bridge, sending a group of five into the water. Rescuers pulled three children to safety.

Witnesses say children tipped into a powerful current when their family's dinghy capsized this afternoon were desperately calling for help as they clung on for dear life.

Stability limitations with the Easy Rider are shared by other boats of the same design, according to the Transport Accident Investigation Commission.