McCarthy (31), a 20-year veteran in the sport, won the men's open Hill Climb title up Signal Hill yesterday in a time of 13min 32sec.
Locky McArthur (Auckland) was second in 13min 38sec and Casper Oxlee (New South Wales) third in 14min 08sec.
McArthur (17) won the junior gold medal and Anja McDonald (Dunedin) won the women's title in 16min 15sec.
It was an exhausting race and McCarthy and McArthur both lay on the grass exhausted when they crossed the finishing line after the tough climb.
"The toughest part was near the end of the steep hill," McCarthy told the Otago Daily Times when he got his breath back.
"It was difficult riding over tree roots when you've run out of power.
"On the steepest pinch, McCarthy got off his bike and pushed it up the hill for 20m before he hopped back on.
McCarthy, a bike mechanic, won his first title at Nelson two years ago. He used his vast experience of 18 years of racing to time his winning bid to perfection.
He was last away at the start, fourth up the last steep hill on the bottom half of the course and did not take the lead until halfway, when the rest of the field was feeling the pinch.
"It always takes me time to warm up he said.
"But I'm consistent and that got me through."
McCarthy, who has never won a New Zealand Hill Climb title, has finished runner-up in the event for the past two years.
McArthur, who was competing in his first Oceania championships, tried to close the gap on his older opponent, but was never able to bridge the gap and stayed between 5m and 10m behind on the second half of the race.
"He was always in front of me when it counted," McArthur said.
"I tried to take him on the steep pitch on the last hill, but it was too tough."
McDonald (29), an industrial designer, added another stripe to her growing championship list in the women's race.
She won the New Zealand cross-country title in 2007 and has spent the past 18 months based in the United Kingdom gaining valuable international experience.
During that time she won the British single speed and the Scottish cycle-cross titles.
"The last 50m pitch to the finish was the hardest part of the race for me," she said.
McDonald grew up in Rotorua and enjoyed windsurfing and snowboarding in her youth, performing with distinction at North Island schools events. She started recreational mountain-biking when she was 18.
Wyn Masters won the first transtasman battle when he beat Haden Lester (Queensland) in the final of the senior men's dual slalom, despite celebrating St Patrick's Day into the early hours of the morning.
Masters (22), a semi-professional bike rider, won the first heat in 15.50sec, the second in 15.62sec for a combined total of 31.12sec.
Lester (22), a Brisbane sales engineer, had runs of 15.86sec and 16.27sec for a combined total of 32.13sec.
Lester made two mistakes when he blew out on the corners in the dusty conditions and it cost him the race.
"The corners were soft and it made cornering difficult," Lester said.
"One little mistake is all that it takes."
Masters made the most of his opportunities to ram home his advantage to take the gold medal from the Australian.
Sarsha Huntington (Queensland) had a comfortable 6sec win over Rita Langley (Christchurch) in the women's final.
Huntington showed remarkable consistency with runs of 18.96sec and 18.87sec for a total of 37.83sec. Langley had runs of 22.22sec and 21.16sec for a total 43.38sec.
It was the first of three gold medals Huntington is chasing at the championships. She is favourite in the four cross, where she is ranked seventh in the world and wants to win the downhill.
"But that will be tough because Harriet [Harper of Blenheim] is fast.
"But I definitely want to win two gold medals from my three events."
Daniel Franks (Christchurch) beat Richard Scott (Dunedin) in the junior men's final with runs of 16.73sec and 15.87sec for a total time of 32.60sec. Scott had runs of 16.97sec and 16.57sec for a total of 33.54sec.
One of the spectacular events of the championships is the four cross that will be held on the Signal Hill course tonight, from 5pm.
