New Zealand maritime authorities are helping in the desperate search for a child missing at sea in the Cook Islands.

The search for the 7-year-old boy has entered its second day after he is thought to have fallen overboard from a ferry near Aitutaki yesterday afternoon.

Cook Islands Police said they were helping in the search and rescue mission for a passenger who fell off the Lady Moana, a local vessel.

"Police confirm the missing person is a 7-year-old boy - a passenger with family en route from [another island] Rakahanga.

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"A large number of local boats are part of the search efforts and Air Rarotonga is also playing a role in the search."

It is understood the boy is a local.

Authorities said their search efforts were being co-ordinated based off a modelled map of the area near popular tourist hotspot Aitutaki.

The NZ Rescue Co-ordination Centre and Maritime NZ were asked for help, a spokeswoman said, after Cook Islands Police were alerted of the situation about 6pm local time. (4pmNZT)

The spokeswoman said they were told once it was discovered the boy was missing, the ferry re-traced its route for several hours until dark.

She said they were supplying drift-modelling information based on where the ferry was located at the time the child went missing.

Weather conditions at the time were "quite rough,'' she said, and their information would help direct vessels helping in the search as to where they should be looking.

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