Of the Tri Nations sides in World Cup action on Saturday, one was said to have the best backline in the world and the other was old and slow.

Who would have thought then that by the end of the day, it would be defending champion South Africa dazzling with a power-packed 49-3 win over Fiji, while Australia's vaunted backs would be smothered in a 15-6 defeat to an impassioned Ireland in the tournament's first major upset.

In the day's first match, Argentina rebounded from an opening loss to England by beating Romania 43-8.

The Irish, who couldn't win any of their four warmup matches last month, outsmarted and outmuscled the Wallabies in the rain at Eden Park to fire up the tournament and shake up the playoff possibilities.

If Ireland hold on to top Pool C, they will likely avoid meeting the Tri Nations sides early in the playoffs.

If No.3 South Africa finish atop Pool D and No.2-ranked Australia finish second to Ireland, the southern hemisphere teams will meet in a quarter-final, with the winner likely to advance to a semi-final with top-ranked New Zealand.

Not only did Ireland rekindle the flame which ended England's Six Nations Grand Slam bid in March, but they also turned into title contenders with their second win from two Pool C matches.

Behind a dominant scrum and fearsome loose forward trio, Jonathan Sexton and Ronan O'Gara each landed two penalty goals and Sexton added a drop goal to inflict the Wallabies' first pool-stage loss since 1995.

James O'Connor kicked two first-half penalties for Australia, who conceded five scrum penalties and struggled to display any fluidity after having lost flanker David Pocock and hooker Stephen Moore to injury or illness in the hours leading up the match at Eden Park.

"It was the performance we knew we had in us," Ireland captain Brian O'Driscoll said.

"We had incredible support and it was our job to give them something to shout about and, thankfully, we managed to do that."

The Springboks impressed for the first time all year.

They'd been criticised for being one dimensional and ageing, and were admittedly fortunate to edge Wales last weekend, so it was a surprise when they uncorked a brutally-efficient performance to nullify the physical Fijians and score six tries without conceding any.

"It was good to get our plan in place and to be able to execute well against a team that was willing to play rugby," Springboks captain John Smit said.

Argentina scored four tries in the first 30 minutes against Romania in Invercargill in the early game, showing plenty of the attacking flair lacking in its opening loss to England.

The Pumas pack was again dominant but, this time, the backline was potent to lead 26-8 by halftime.

Santiago Fernandez, Juan Manuel Leguizamon, Juan Figallo and Lucas Gonzalez Amorosino, voted man of the match, scored the first-half tries, converting Argentina's glut of possession.

In the second half, replacements Juan Jose Imhoff and Genaro Fessia scored tries. Centre Martin Rodriguez kicked five conversions and a penalty for 13 points.

Winger Iionel Cazan scored the lone try for Romania, while Tiberius Dimofte kicked a penalty.

Meanwhile, US captain Todd Clever escaped suspension when he was cleared of a dangerous tackle charge in the win over Russia.

The ruling means Clever is available to play against two-time champions Australia on Friday in Wellington.

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