Hansen: Sonny Bill didn't know the rules

All Blacks coach Steve Hansen backed Sonny Bill Williams after his error against France, saying his star had no idea about the rule that led to his yellow card.

New Zealand held off the hosts at the Stade de France to claim a 38-18 win in their Test on Saturday.

But the All Blacks stuttered early in the second half, when Les Bleus were awarded a penalty try after Williams inexplicably deliberately slapped the ball over the dead-ball line.

However, Hansen defended the 32-year-old, saying Williams was unsure about the rule thanks to his experience in rugby league.

"The Sonny one was he doesn't know the rules because he was playing league and in league you're allowed to smash it over the dead-ball line," he told a news conference.

"I got a lesson myself from Fozzie [assistant coach Ian Foster] that once you've committed an offence in the goal area then it's like you're not there, they make you invisible and they felt the French man was going to catch the ball so it was a try. You can't do too much about that.

"It was a good lesson for Sonny, he'll know next time to catch it rather than bat it over the line."

The All Blacks had been cruising, taking a 31-5 half-time lead before France rallied to get within 13, but New Zealand closed out their win.

Hansen said his side's ill-discipline almost proved costly, but he was happy with the way his team dealt with the pressure put on by the hosts.

"I think we lost our discipline a little and as a result of that we allowed the momentum to shift and the French were good enough to take that opportunity and put us under pressure," he said.

"Having said that, I was very happy with the way we fought our way through it and scored a good try at the end.

"We scored enough points in the first half to get the job done."