NIWA is forewarning Kiwis with Cyclone Tino forecast to send ceiling-height swell against the eastern North Island next week.
With peak waves of 8m at its centre, the massive offshore storm is growing even larger with even lower air pressure as its moves towards New Zealand.
"This new deepening cold-centred low is what will be helping to create bigger waves and rips around eastern NZ in the days ahead," WeatherWatch warns.
NIWA has issued a graphic showing the forecast wave height and wind direction with swells set to smack New Zealand.
"If you're an eagle-eyed surfer you may have been keeping a watch for the potential of cyclone generated swell this week," it says on Facebook.
"Monday and Tuesday are set to bring the best conditions to catch a clean wave or two with some lingering waves on Wednesday.
"If you're heading to the beach with little ones next week keep an eye on the bigger waves and stronger rips."
WeatherWatch warns the most impacted areas will be the East Cape, Gisborne and Hawke's Bay.
"Many populated holiday beaches in the eastern North Island will also be affected by an increase in wave heights, wave frequency, rips and currents such as Bay of Plenty, eastern Coromandel and parts of Northland and Wairarapa," it says.
"This offshore system may make for some surprisingly rough patches in some of our eastern beaches."
WeatherWatch warns beaches that are normally calm in summer may have swept by waves up to 3m tall.
"These offshore storms can be very dangerous to beachgoers, especially when the weather on land is likely to be hot and sunny at many beaches - masking the new risks forming in the water," it says.
"While increased swells may not be that great compared to the more rugged West Coast storms even an additional metre or so of wave height could create much more difficulty swimming with younger and older people and weaker swimmers etc, especially in beaches which may traditionally been seen as 'safe' and 'calm'."