New Zealand assistant coach Ian Foster accepts that striking the balance between physicality and strong discipline will be vital in Saturday's Test series decider against the British and Irish Lions.

The All Blacks were left reeling by centre Sonny Bili Williams' red card as they lost the second Test 24-21 in Wellington last weekend.

The Lions were also guilty of conceding several penalties, and they had prop Mako Vunipola sin-binned for an illegal challenge on New Zealand fly-half Beauden Barrett.

Williams is now serving a four-week ban following a judicial hearing, and Foster said: "We've always said that we just go through the processes and stand by the decisions. They are what they are. We've copped a punishment for it, and we move on."

Asked about the balance between physicality and discipline, Foster added: "That is the line, isn't it?

"That is the beauty of these sort of games where it is a deciding Test. You know there is a lot at stake and everyone is trying to impose themselves physically, and it is whether you are smart enough to control that and be effective with it.

"It is something we work hard at.

"We have to make sure we are totally under control, but bring the physicality. The two Tests have been quite noticeable - one we won that battle, and one they won that battle. It shows how important it is.

"We probably didn't do a good enough job when we went down to 14 to make sure our mindset didn't narrow, and I think that it did. There was still some clear stuff for us to go to, and probably our vision was a bit narrow.

"That is not something natural for us, but we allowed ourselves to get into that sort of mode. For 45 minutes and a man down, that mode was working pretty well for us, but when we needed to see things a bit differently we probably took too long to do that."

Foster, meanwhile, said centre Ryan Crotty, who suffered a hamstring injury during the first Test, is unlikely to recover in time. And with Williams also sidelined, All Blacks head coach Steve Hansen seemingly faces a choice between Ngani Laumape and Malakai Fekitoa as Anton Linenert-Brown's midfield partner.

The Lions will arrive in Auckland on Wednesday after time spent resting and recuperating in the Southern Alps' tourist haven of Queenstown.

"Everyone has their own different way of doing things," Foster said. "We like to switch on and switch off as well.

"You have to factor their decision into the fact they have been here for a decent amount of time on tour. This is our fourth week together. We trust our routines, they probably trust theirs."

Asked to comment on Lions head coach Warren Gatland's post-second Test assertion that New Zealand have yet to "stress" the Lions, Foster added: "I am not surprised he is having a relaxing week in Queenstown, then, if that's what he thinks."

All Blacks flanker Jerome Kaino, meanwhile, says that the third Test showdown feels like a World Cup final.

Kaino helped New Zealand to World Cup glory in 2011 and 2015, starting the finals at Eden Park and Twickenham.

And he now returns to Eden Park, where the All Blacks are unbeaten for 39 Tests, with a pulsating series on the line.

"With it being 1-1, it does have the feeling of a World Cup final, so I am getting excited about the prospect of being able to play," Kaino said. "It definitely has that feel. I think the excitement we had at training (on Wednesday) backs that up.

"The Lions are a great side, and there is a lot of history between these two teams.

"There has definitely been a bit of edge there (in training), but the guys are excited."

Kaino had an unexpected view of the second Test, watching 55 minutes of it from the sidelines after he was taken off and replaced by back Laumape in an attempt to compensate for Williams' red card.

"As you saw last weekend, both teams were quite keen to get amongst it," he added. "As we saw, discipline is a huge part of the game.

"We are not going to go in there and be cautious about anything. We have to impose ourselves physically like the Lions did, and that's definitely an area we want to improve on.

"We know a lot of things are spoken about in the week, but that's not a reason for us to hold back. We know we can play physically within the laws.

"There is always going to be feeling between these two teams. When we cross the white line we throw it out there, but once it's over, we dump it."