I agree with cagney regarding they were in the wrong spot.
I've sailed that route
a few time and pretty much every sailor who sail to/from NZ checks out Bob Mcdavitt's weekly weathergram which can be found here.
WEATHERGRAM: July 2010
On the 25th of July Bob McDavid posted the following
Convergence zone SPCZ is mainly along 8 to 11 South from
Solomons to Tuvalu/Tokelau. It occasionally peels away to the southeast
and makes a trough-an example of this should occur over Tonga Niue
Sat 31 July, and this may develop into a LOW to south of Southern Cooks
early next week."
In a well written lat 38 artical titled "Nightmare Off New Zealand" which looked into the death of four sailors and 4 lost/destoryed boats during a 20 day period contained this telling quote
"The one condition we knew we had to avoid was getting caught between a high and an approaching low — especially a low coming down from the north."
The artical can be found here.
Latitude 38 - Nightmare Off New Zealand
IMO the main catalst was the weather
pattern, rather than the any design flaws in the catamaran
I suspect a monomaran which was flying a similar amout of canvas
would have encountered major problems aswell.