New Zealand set up a mouthwatering Tri-Nations decider against Australia in Sydney next weekend with a convincing victory over South Africa on Saturday.

Andrew Mehrtens returned to the fold in a much-changed All Blacks line-up and his influence on the game was telling as he slotted over four penalties and two conversions.

The conditions were hardly conducive to running rugby, as the knock-on count testified, and Mehrtens won both the kicking and tactical duel with South African opposite number Butch James.

By the time Pita Alatini evaded the desperate South Africa defence to give New Zealand the lead just seven minutes in, the All Blacks had already broken the gain line on a number of occasions.

South Africa looked a solid unit, as they have throughout the competition, but nothing more, and once again they failed to break down the All Blacks defence. In Cape Town last month, a second-half spent continually on the attack failed to yield a try - they have scored just two in four Tri-Nations games - and they rarely threatened.

There has been much talk that coach Harry Viljoen has turned the corner with the Springboks and they have produced some sterling displays this tournament, not least against Australia. But on Saturday's evidence there is still some way to go. They relied on the boot of Van Straaten, who was possibly playing in his last international after being linked with a move to English side Leeds Tykes, for all 15 points.

After the break New Zealand increased their dominance, largely through Mehrtens. He landed a 55-yard penalty 13 minutes after the interval and it was Mehrtens' break soon after that forced Springbok hooker Lukas van Biljon to tackle full-back Leon MacDonald when a try was certain.

Australian referee Peter Marshall had little option, a penalty-try was awarded and Mehrtens slotted over the afters. Van Straaten managed another long-distance kick and despite a late rally, New Zealand held on to move top of the Tri-Nations table.