The NWIH extreme enduro is a stand-alone event but also doubles as the fifth and final round of the annual KTM Extreme Enduro Series, a competition that runs separately but parallel to the enduro nationals.
"There will be some pretty hard sections to test the riders but it's actually the weather that may play the biggest part in how tough the course becomes," said Clarke.
"The course won't be impossible but the riders will certainly have to be on their toes just to finish it."
Kiwi international Clarke is a multi-time New Zealand champion and a four-time medallist at the Olympic Games of enduro racing, the International Six Days Enduro.
A prologue race on Sunday determines the start order for Monday's main event, where riders will be started a minute apart.
The current national enduro champion and points leader after four of six rounds in this year's championships, Auckland's Chris Birch, is clear favourite to win both the nationals round and the NWIH event at Oparau, but three-time national cross-country champion Adrian Smith might have something to say about that, the man from Mokau having stolen Birch's thunder in winning the previous round of the enduro nationals near Christchurch a fortnight ago.
Glen Eden's Birch was winner of the inaugural NWIH event, the only one to finish unaided, in a time of 3 hours 12 minutes. Hokianga's Mitchell Nield was 2 hours behind in second place. Following Nield, in finishing order, were Taupo's Mark De Lautour, Mokau's Smith and Marton's Cam Smith (no relation), who each took more than eight hours to complete the race.
Kiwi international Birch also won last year's event and rates as favourite to collect the trophy this weekend too, although last year's NWIH runner-up, Hawkes Bay's multi-time national moto trials champion Warren Laugesen, last season's KTM Extreme Series winner Mike Skinner, of Auckland, and seven-time and current national moto trials champion Jake Whitaker, of Wainuiomata, can't be discounted.
Other experienced NWIH riders such as Nield, De Lautour, the two Smiths and Tauranga's rising star Jim Lowe-Pattie should also threaten to steal Birch's glory.
A rider's skill, strength and stamina will be stretched to the limit and bike reliability pushed to the brink, but luck may also play a huge part in this least heavenly of all off-road events.