Just one tropical cyclone has struck the Southern Hemisphere so far this season - the fewest in nearly 60 years.
On average the region has had five named storms by Christmas Day.
But so far there has been just one - Dahlia - which formed south of Indonesia late November.
Niwa expects eight to 10 tropical cyclones to hit the southwest Pacific this season.
Northern New Zealand may see at least two tropical cyclones making landfall in coming months.
Niwa says on average, at least one ex-tropical cyclone passes within 550km of New Zealand each year.
They have an equal chance of heading east or west of Auckland and the North Island.
In April, the remnants of two cyclones caused widespread flooding and damage across the North Island and eastern regions of the South Island.
New Caledonia, Vanuatu, Fiji and Tonga are expected to be hit by at least four cyclones before April. At least four storms are expected to reach at least Category 3 and two may rise to an even more destructive Category 4.
Niwa is warning Category 5-strength cyclones with average wind speeds of 196 km/h or greater have happened in past seasons that have had similar weather patterns and all communities in the regions should remain alert and well-prepared for severe storms.
The most intense tropical cyclone to hit the hemisphere was Winston, which devastated small island nations including Fiji, Tonga, and Niue two years ago.
As with most years, activity is expected to increase in the late part of the tropical season from February through to April.
Niwa meteorologist Ben Noll said sea temperatures are much warmer than usual so New Zealand needed to be vigilant, especially for the later part of summer.
Experts looked for unusual rising or sinking motion in upper levels of the atmosphere.
"As we go into the new year we have to keep watch. The warm seas are fuel for powerful weather systems," said Noll.
He said the first half of summer was the best time to holiday, as the second half was more likely to be hit by storms and rain, especially in the North Island.
Meanwhile the year of extreme weather continued in recent days when Christchurch recorded its highest Christmas Day on record hitting 30.9C and Milford Sound was drenched with 70.6mm of rain.
Boxing Day saw Wellington inundated as a sudden burst of rain saw more rain fall in an hour than the previous 48 days preceding it. Wellington Airport recorded 15.8mm of rain in 60 minutes turning streets into waterway and firefighters called to pump out flooded shops.