The roar of jet boat engines will fill the air near Te Anau tomorrow as the 2021 ITM NZ Jet Boat Marathon gets underway.

Twenty race teams are set to compete in the week-long event, hitting speeds of up to 200kmh on Southland and Otago rivers.

Racing begins on the upper Waiau at 3pm tomorrow, following technical inspections of the boats at the Fiordland Community Events Centre car park from 8am (or the Real Journeys workshop if raining).

"They're big horsepower motors in the boats," says River Racing NZ past president Malcolm Mclean.

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"Some have got the turbines out of helicopters in them, so they're very fast.

"But it's a test of endurance really. They race about 900km over the week, so who's still going in the end."

The unlimited class machines are the fastest, up around 1000 horsepower. Four are signed up to race in the marathon, including renowned Winton racer Roger Preston.

"I've been in one at 126mph (200kmh), so it's a commitment," Mclean says.

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"The guys have been going up and down the rivers in the last few weeks, pre-boating, familiarising themselves with it.

"At that speed, everything's happening fast."

Mclean says it's an expensive hobby, but well supported none the less. And the FX and CX Class, which have engine-size restrictions, are good for entry level racing.

"There's some hot competition in those classes. They battle away, there's only normally a couple of minutes separating the whole field. It's good to watch, especially the end."

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The secret is a good set up, with hull and motor combination, and then picking good race lines and making good decisions on the day.

"We raced on the Waitaki earlier in the year, and it dropped 100 cumecs overnight, so where all the guys had their race lines and journals and the rest of it, there was no water there."

Racing tomorrow will be from Lake Manapouri up to about 1.6km below the Te Anau control gates.

Sunday will be on the lower Waiau, starting a Tuatapere and racing up to the Red Cliff stream confluence, near Manapouri, and then back down.

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That's the longest leg of the whole race, about 81km.

After that, racing moves on to the Oreti, Mataura, Dart, Clutha and Matukituki rivers, finishing next Saturday, October 26.

"We get a good crowd of spectators, a really good following, and have cut some of the legs to make shorter runs, so they don't have to wait so long."

The marathon is held every year, sometimes down here, sometimes in North Island.

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"We're down a wee bit on previous years, due to Covid restrictions and the financial situation because of Covid, but it's good to still have enough to run it."

Other Southland racers include Damien Pulley, from Winton, and his navigator Paul Cross, from Gore.

SSS Engineering - Unlimited

NZ1 John Derry, 366 Mike Pooley, 301 Rob Pooley, 367 Roger Preston, 377 Tom Kelly

Heli Ops Southland - A Class

321 Ant Holland, 36 Damien Pulley, 248 Justin Hill, 627 Rob Donald

Keelowcraft - CX Class

99 Andrew Scott, 121 Byron Campbell, 70 Callum McKenzie, 28 Craig Robinson, 666 David Robinson, 841 Grant Perry, 185 Richard Foster, 305 Steve Rapsey

Majestic Housing - FX Class

349 Greg Wilson, 979 Lee Harrison, 166 Riley Smith