New Zealand coach Steve Hansen has allayed any fears over an injury to captain Richie McCaw ahead of their Rugby World Cup clash with Tonga.

Openside flanker McCaw has been left out of the All Blacks' final Pool C match at St James' Park on Friday due to a thigh problem that Hansen insists is minor.

The defending world champions are understandably heavy favourites to beat Tonga and clinch top spot in the group, leaving Hansen unprepared to risk his skipper ahead of the knockout stages.

"If we were playing a final, he would be fine, but we are just not prepared to take any risks with anyone that is not 100 per cent," Hansen said.

"He got a bang on his thigh and it's just really sore when he stretches out at top-medium pace, so it is just not worth risking it.

"We've had plenty of gasps from home before, but we're okay with it and the people that are anxious about it don't need to be anxious. If this was the quarter-final, he would be playing.

"We've got enough depth to be able to say, 'hey, we don't have to risk people if they're not 100 per cent'."