Ussher finished in eighth place feeling a "little bit disappointed" with his performance.
The field was blown apart early by the strong swimming trio of Pete Jacobs (Australia), eventual winner Rasmus Henning (Denmark) and Enako Llanos (Spain).
They gained a commanding lead of more than 5min on the rest of the field.
Jacobs led out of the 3.8km swim in the Main-Donal Canal in 46min 21sec, while Ussher was nearly 8min behind in 54min 37sec.
Ussher, of Nelson, was third in Roth last year and had hoped to perform well in the swim after spending what he described as a disproportionate amount of time in the pool and feeling encouraged by his progress.
But his swim this year was slower by 4min than last year, leaving the multisporter with plenty of food for thought.
"It is getting hard in iron-distance now for people who aren't really good swimmers, especially with ITU guys coming through," Ussher told the Otago Daily Times after the race.
Before the race, Ussher said he needed a 48min swim to help keep him in touch with the top cyclists.
Making up a 5min or more disadvantage in the cycle leg was always going to be very hard when the rest of the field were strong cyclists, he said.
"It always is a little bit disappointing when you don't perform as well as you would like.
"But that is just the reality of sport.
"People put their whole year of training into one race and sometimes it doesn't come off."
Ussher has had a big racing programme this northern hemisphere season and still has more races to come.
Asked if he could continue to pursue long-distance triathlon, he said he would just take each race one at a time.
He does not have another one planned this year and does not know yet whether he will defend his Challenge Wanaka title or do the Lake Wanaka Half instead in January, although he expects to return to Wanaka for the Challenge Festival.
"One thing the ironman distance has done is it has really focused my motivation for off-road [events].
"I appreciate how much I enjoy getting off road racing," Ussher said.
The conditions yesterday were much cooler, in the mid-20degC range, after rain on Sunday relieved the heat wave being experienced in Germany.
Henning won the race in 7hr 52min 46sec, with Sebastian Kienle (Germany) second in 7hr 59min 6sec.
Llanos was third in 8hr 2min 33sec, while Ussher finished in 8hr 24min 56sec.
Women's world champion Chrissie Wellington, of Britain, stunned the large crowd of spectators by finished seventh overall, in a world-record time of 8hr 19min 13sec, smashing her world record set last year at Roth by nearly 20min.
The second woman home was Rebekah Keat (Australia) in 8hr 52min 10sec, and Tereza Marcel (Czech Republic) was third in 9hr 9min 29sec.
Jodi Currie, of Queenstown, was second in the women's 25-30 year age group, finishing her race in 10hr 12min 16sec.
The veteran team of Shirley Rolston, Ray Bennet and Stan Gawler, all of Christchurch, did not finish after Gawler (80) withdrew after 15km into the marathon because he did not feel well.
By race day, entries had exceeded 5000 athletes, with more than 3000 individuals and 600 teams.
It was the largest in the event's nine-year history.
