Yokohama - While happy the All Blacks opened their Rugby World Cup campaign with a 23-13 victory over arch-rivals South Africa, coach Steve Hansen faulted the effort and warned they had to improve to win the Webb Ellis Cup.

The All Blacks set winning a third consecutive world crown, and fourth overall, as their goal with the ultimate target of regaining domination of the sport.

But in their opening Pool B match against South Africa, they struggled to get going until a golden patch midway through the first half when they scored their two tries in the space of five minutes.

The only other try in the match was scored by the Springboks in a failed bid to overhaul the All Blacks in the second half although they were on the verge of nailing a second until flying wing Cheslin Kolbe was cut down by Richie Mo'unga just short of the line.

Hansen described that tackle as "a match winner", adding "that's what rugby's about, to try and win the moments that matter".

But while pleased with the defence, as well as the low penalty count, Hansen found areas of the All Blacks game that needed improvement, particularly in the forwards.

"We won, so you have to be happy with that. Were we perfect? No, but you're never going to be at this stage of the tournament," he said.

"There's plenty of stuff we can work on.

"It was really hard early in the game for us. Our set piece wasn't as nice as you'd expect it to be and therefore you can't have a platform to strike off.

"The boys adjusted okay and as they game wore on they've got more fluid with it, but it's always going to be a titanic struggle and it's an area we're just continually working on."

New Zealand's next game is against Canada on Wednesday in a pool which also includes Namibia and Italy.

Scorers:

South Africa

Try: Pieter-Steph du Toit

Conversion: Handre Pollard

Penalty: Pollard

Drop goal: Pollard

New Zealand

Tries: George Bridge, Scott Barrett

Conversions: Richie Mo'unga (2)

Penalties: Mo'unga (2), Beauden Barrett

Teams:

South Africa

15 Willie le Roux, 14 Cheslin Kolbe, 13 Lukhanyo Am, 12 Damian de Allende, 11 Makazole Mapimpi, 10 Handre Pollard, 9 Faf de Klerk, 8 Duane Vermeulen, 7 Pieter-Steph du Toit, 6 Siya Kolisi (captain), 5 Franco Mostert, 4 Eben Etzebeth, 3 Frans Malherbe, 2 Malcolm Marx, 1 Steven Kitshoff 

Substitutes: 16 Bongi Mbonambi, 17 Tendai Mtawarira, 18 Trevor Nyakane, 19 RG Snyman, 20 Francois Louw, 21 Herschel Jantjies, 22 Frans Steyn, 23 Jesse Kriel

New Zealand

15 Beauden Barrett, 14 Sevu Reece, 13 Anton Lienert-Brown, 12 Ryan Crotty, 11 George Bridge, 10 Richie Mo’unga, 9 Aaron Smith, 8 Kieran Read (captain), 7 Sam Cane, 6 Ardie Savea, 5 Scott Barrett, 4 Sam Whitelock, 3 Nepo Laulala, 2 Dane Coles, 1 Joe Moody

Substitutes: 16 Codie Taylor, 17 Ofa Tuungafasi, 18 Angus Ta’avao, 19 Patrick Tuipulotu, 20 Shannon Frizell, 21 TJ Perenara, 22 Sonny Bill Williams, 23 Ben Smith