All Blacks centre Sonny Bill Williams is under an injury cloud ahead of the Bledisloe Cup decider, but the midfielder's fitness is the least of New Zealand's concerns in the wake of their 27-19 loss to the Wallabies.

All Blacks coach Steve Hansen revealed Williams fell heavily on his tailbone about 20 minutes into the Rugby Championship decider at ANZ Stadium and was still sore the day after the test. He's unlikely to train with the team on Tuesday but not ruled out of next Saturday's test at Eden Park just yet.

"He landed on his rear end... got a knock on the buttock and that's tightened up so we'll have to see how he goes," Hansen said.

All Blacks v Wallabies: What did we learn? Share your stories, photos and videos. Contribute

Williams' injury goes some way to explaining his hesitation to take contact in Sydney, but in reality a fit again Ma'a Nonu seems an obvious selection at second five after the All Blacks failed to win the crucial collisions in the middle of ANZ Stadium.

"Ma'a's fine and Sonny could still come right," Hansen said. "We'll just wait until we get through to Tuesday and make some assessments then."

There was better news on fullback Ben Smith who knocked himself out unsuccessfully trying to stop Wallabies wing Adam Ashley-Cooper from scoring in the second half.

"Ben is good. Even after the game he was tested and got 30 out of 30 in his [concussion] test and this morning he's not suffering from anything," Hansen said. "We'll do what we always do, we'll do all the tests and we won't play him if he's not right."

READ MORE:

* 'Bitter-sweet' debut for Milner-Skudder

* Wallabies set sights on Bledisloe Cup

* Wallabies end All Blacks win drought

* Hinton: Milner-Skudder provides cheer

* Recap: Wallabies win Bledisloe opener

* Robson: Who will star at the World Cup?

Hansen made it clear he would be picking his strongest side in a bid to keep the Bledisloe Cup in New Zealand and avoid losing two tests in a row since back to back defeats to South Africa and Australia in 2011.

"From a selection point of view it's been critical we've been doing what we're doing, but we've said the Bledisloe, outside of the World Cup, is the most important thing to us, so we have to really batten down the hatches now and make sure we get the next week right."

It promises to be an uncomfortable week for some players with Hansen subjecting himself to a late night video-nasty after returning to the team hotel on Saturday night. "It doesn't get any prettier. There are a lot of things we can improve on. The big thing is we can't afford to panic. We have a plan that we are playing too and this week we have to make sure we get the critical few things right and adjust one or two things we are doing and get better at it.

"There will be some people thinking I can do better than that. The people who missed the tackles are usually pretty reliable and they'll be hurting and so they should be."

That reference was no doubt to Dan Carter and Kieran Read, who both fell off crucial tackles they would normally nail. Blindside Jerome Kaino said there were some disappointed individuals after the loss, but hinted that a lack of time together as a combination this season had played its part in the defensive slips.

"I think it's a matter of the group being able to be in the trenches a lot more together. A lot of those misses were individual errors, guys not getting up and making their tackles. But we'll address that this week and it will make for a much improved performance at Eden Park."