He and co-driver Paul Barnes have extra motivation after they weren't really a factor in this season's national title chase, due to having to miss the opening round as Auret recovered from a broken leg.
"Hamish wants to get a win this time, that's the thing there," said Cowper.
"Jarred's got his 900hp truck.
"But there's no favourite. Everyone's likely to take it out, all ten of them."
Drivers will start the first obstacle in the order of placings from the national series, as in Scott Biggs ahead of brother Jarred, and then for the next course Jarred will lead off while Scott moves to the back of the queue and the drivers will continue to rotate accordingly.
"It's probably the toughest we've ever had, a technical course," said Cowper, who will be acting as co-driver for Palmerston North's Stuart Earle.
"I brought a digger this year so carved it out quite a bit.
"Because it's the top 10 drivers, we can make the courses pretty tough and they can get themselves out of tricky situations."
Unlike the national series, where the goal is to make the smallest score by avoiding penalty points, here the idea is to make the largest score with 1000 points available.
Nine of the obstacles are divided into 10 'gates', with drivers getting 100 points for each gate they clear.
The other obstacle is based on the fastest time, and in the event of a tie at the end of the day, it will be used again to decide the overall winner, as happened last year when Scott Biggs won the challenge ahead of Tauranga's Andrew Garner.
Cowper's devious mind has created some tough obstacles this year, such as jumps which could make the truck's corkscrew if they drive at a certain angle, or a steep climb that requires full power but is immediately followed by a vertical drop.
"If they don't think about it properly, they're going to come to grief," he said.
"It's as even as we're ever going to get it."
The advantage of a ten truck field, as opposed to 40 trucks at national rounds, is that the obstacle conditions remain virtually the same, rather than getting worn down by the time the last drivers attempt it.
The spectators have a good view of the whole course from the adjacent hillside, while there will be commentary and an electronic scoreboard to keep them informed of the points standings.
The Extreme Challenge starts at 10am at the Cowper's Turakina property on State Highway 3.
Entry is $20 for adults or $50 for families.
The action will be filmed and later screened on two 60-minute episodes of TV3's CRC Motorsport.
The drivers are
Scott Biggs (Auckland); Jarred Biggs (Auckland); Derek Smyth (Manawatu); Hamish Auret (Whanganui); Stuart Earle (Palmerston North); Neville Mather (Waikato); Rowan Huckstep (Taranaki); Russell Luders (Hamilton); Dave McKie (Waikato); Greg McDell (Waikato).