Surf lifesaving officials are casting a wary eye over weather forecasts before the biggest event on the national calendar in Mount Maunganui next week.

Nearly 1600 athletes will compete in the four-day New Zealand national championships, sponsored by TSB, from March 21-24, representing 45 clubs from around the country.

It's one of the largest turnouts for the championships for the best part of a decade, coinciding with the event being back at Mount Maunganui for the first time since 2013, but event manager Chris Emmett has tempered his delight at the numbers by spending several nervous days poring over various internet meteorological sites.

'There's a fair bit of activity up in the tropics coming our way and the forecasts have been changing drastically over the last few days - at this stage, it's quite literally up in the air whether it will hit Bay of Plenty earlier or later in the week,” says Chris.

'We've had an incredible summer so far and we'd love for it to continue for one more week at least.”

At least two tropical depressions are due to form over the weekend - one over the Coral Sea and one to the east of New Zealand - although recent forecasts suggest the worst of the weather will slide into the Tasman Sea.

'What is likely is that we won't have a shortage of waves for the event, which is exactly what the competitors want and the key skill being tested at the championships.

'The trick is to balance those surf skills with getting through all the additional heats and semifinals we'll be running, while making sure we're keeping our athletes and officials as safe as possible.”

Numbers in the under-16 division have jumped sharply this season, with more than 150 competitors due to line up in both the under-16 male and female board race heats.

There are 137 girls lining up in the under-16 diamond race, while Emmett is also delighted with a lift in the open women's division, where 59 females will tackle the ski race and 43 the ironwoman.

Most of the New Zealand representative Black Fins and junior Black Fins squads will compete, along with former Olympians like Steve Ferguson and Teneale Hatton.

While perennial heavyweight clubs Red Beach, Mount Maunganui and Mairangi Bay have nearly 90 competitors each, the likes of Omanu, Orewa, Piha and Paekakariki also have broken through the 50-athlete barrier.

Added to that is a sprinkling of smaller clubs; Northland's Waipu Cove and Southland's Oreti will both send squads laden with canoe crews, with Oreti joining a number of South Island clubs in making the annual pilgrimage north.