Significant surges, coastal erosion and closed boat ramps are among the effects of the Tongan volcano eruption on the Bay of Plenty's East Coast.
The Thames-Coromandel District Council said wave periods of 12-30 seconds had been observed.
Twelve seconds is considered a "high energy" surge.
"High energy waves can be dangerous" regardless of their height, said the council's emergency management manager Garry Towler.
"People should avoid going out in this."