DUNEDIN, New Zealand (AP) — The Waikato Chiefs opened their Super 15 title defense on Friday by winning a bonus point in a 41-27 victory over the Otago Highlanders despite starting as underdogs, shaking off any skepticism about their credentials for the championship this year.

The ACT Brumbies later extended their strong start to the season, also earning a bonus point in a 30-13 victory over Australian rivals the Melbourne Rebels. Morne Steyn kicked 20 points as the Bulls beat the Stormers 25-17 to open the South African conference.

Doubts had arisen before the Chiefs' match that Waikato could emulate the heights of last season in winning the title for the first time in the 17-year history of the southern hemisphere championship. While several of their leading opponents — including the Highlanders — had gained in strength, the Chiefs recruited sparingly and entered the season-opener with a host of key players lost through injury.

The Highlanders fielded a lineup that included veteran All Blacks prop Tony Woodcock in a strengthened pack and All Blacks center Ma'a Nonu at the heart of a sharpened backline. The Chiefs were forced to field a lightweight center combination in the absence of last season's star pairing of Sonny Bill Williams, who has returned to rugby league, and the injured Richard Kahui.

But it was the Chiefs' lineup, especially pint-sized center Tim Nanai-Williams, which outshone the Highlanders in a match in which seven tries were scored and the lead changed on six occasions.

Nanai-Williams scored the first and last of the Chiefs' four tries with displays of exceptional pace. As the match swept from one end to the other of Dunedin's enclosed Stadium, all seven tries went to outside backs.

Wingers Asaeli Tikoirotuma and Patrick Osborne also scored for Waikato, while fullback Ben Smith and wingers Hosea Gear and Kade Poki touched down for Otago. In one particularly frenetic passage, 15 points — including two tries — were scored and the lead changed three times in three minutes immediately after halftime.

"Those (outside backs) were pretty electric," Chiefs captain Craig Clarke said. "Across the board we'd be a little bit disappointed at letting in a few tries. But we've got a hell of an exciting team and we can definitely score some points."

Having been dropped as the Springboks' first-choice flyhalf last season, Steyn began 2013 in ideal fashion, making seven from seven kicks at goal as the three-time champion Bulls won at home over local rivals the Stormers. Steyn's accuracy was the difference as Elton Jantjies — one of the men trying to replace Steyn as the regular South Africa No. 10 — had an awful start for the Stormers. He missed his first four kicks on his Super 15 debut for the team and was pulled off the field by coach Allister Coetzee.

Jean De Villiers scored a 50th-minute try for the Stormers to help them close a 12-0 deficit to 12-10, but hooker Chiliboy Ralepelle pounced on a loose ball after a deflected chip for the Bulls' only try. A late score by Stormers lock Andries Bekker made little difference in the end, with Steyn's unerring accuracy decisive.

"Back to the drawing board. There's going to be some hard words," said Stormers coach Coetzee, who has led his Cape Town-based team to a final and two semifinals in the last three seasons but still hasn't won the title.

At Melbourne, Brumbies fullback Jesse Mogg scored a try and set up Clyde Rathbone's first five-pointer since returning from a four-year break in a come-from-behind win.

The Rebels led 13-12 at halftime but were starved of possession after the break, allowing the Brumbies to dominate with 18 unanswered points.

Mogg opened the scoring in the fourth minute after Melbourne flyhalf Kurtley Beale turned over the ball when he was tackled midfield. Mogg then set up the Brumbies' second try for winger Rathbone.

Fullback James O'Connor set up the Rebels only try for Scott Higginbotham in the 18th minute and added two conversion and two penalties to give the home side the lead at the break.

The second half belonged to the Brumbies as captain Ben Mowen crossed in the 61st minute and substitute Robbie Coleman scored the fourth try for a bonus point in the 74th. Center Christian Lealiifano contributed two conversions and two penalties for the visitors.

The Rebels' woes were compounded by fitness concerns for their two star backs. O'Connor had a suspected concussion while Beale injured a shoulder while making a tackle.

The Brumbies are now undefeated from two rounds after having downed the Queensland Reds 24-6 in their opening match last week.