Eleven people have been rescued after their waka capsized in three metre swells at Tolaga Bay, north of Gisborne.
The group, who were practising for the upcoming transit of Venus celebrations on Saturday, could have drowned, police told Radio New Zealand.
The double-hulled traditional Maori canoe tipped as it tried to get through the heavy seas at the mouth of the Uawa River.
None of the group was wearing a life jacket.
Gisborne Senior Sergeant Maui Aben says most were in the water for more than an hour and were suffering from hypothermia when they were rescued.