About 50 members of Ngāti Kahu are preventing people launching boats at Taipā, where the competition will be based on Saturday and Sunday.
The group says it was protesting over-fishing and a government proposal to introduce a bill redefining the principles of the Treaty of Waitangi.
Up to 500 people were expected to take part in the annual contest.
Five police officers were understood to be present at the boat ramp.
Police said they were monitoring the event.
RNZ
