Maori are claiming customary title to 21 beaches around the country under the Marine and Coastal Area Act, it's been reported.

3News says it has documents detailing the claims - and another by three competing groups of Maori to the entire coastline around Motiti Island in the Bay of Plenty.

A separate claim has been filed for whitebait rights in southern Taranaki under the Marine and Coastal Area Act, 3News says.

NZ First leader Winston Peters, who opposed the law when it was passed by parliament, says he's been proved right.

"I predicted it would set Maori against Maori, and Maori against Europeans and the rest of the population - it's going to be the disaster we forecast it would be," he said.

The previous Labour government passed the Foreshore and Seabed Act which put all the coastline under Crown ownership, so no one could claim title to it.

After National came to power in 2008 it replaced the Foreshore and Seabed Act with the Marine and Coastal Area Act and changed the law, giving iwi and hapu the right to have claims to customary title heard in court or settled through negotiation with the government.

Customary title gives them limited development rights but they can't deny beach access to anyone and they can't claim freehold title.