All Blacks star Sonny Bill Williams admits his side didn't show Argentina enough respect in the second half of Sunday's 36-10 win in Buenos Aires.

Heading into the sheds 29-3 ahead, the All Blacks struggled to nail the basics in the second stanza and gave away frequent turnovers and penalties.

In the end, only some impressive last-ditch defence prevented Los Pumas from scoring more than their single try, via No.8 Juan Manuel Leguizamon.

David Havili then scored on his All Blacks debut after the siren.

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Chastised by former Test boss Laurie Mains last week for being past his prime, Williams responded with a tidy defensive display and appeared in good nick.

He made nine tackles and two offloads.

But he admitted the world champions let their focus slip after wrapping up the victory - and the Rugby Championship - before the half-time hooter.

"We probably didn't respect them enough, 29-3 up. That's when we have to nail our processes and scale our game back," the 32-year-old Williams said.

"But we've got to enjoy it - we won the Championship.

"Let's give credit where credit's due, Argentina came back."

With the Championship secured for the fifth time in six seasons, the All Blacks may be tempted to rest some players for next week's clash with South Africa.

They will send six players home from Buenos Aires, with a sextet including Sam Whitelock, Brodie Retallick and Sam Cane to meet them in Cape Town.

Irrespective of Steve Hansen's squad selection, Williams expected a challenge from the Springboks, still smarting from their 57-0 obliteration in Albany.

"We can come with all the players we want but if we don't turn up with the fire and passion those guys are going to bring, it's going to be tough," Williams said. "They're going to be hurting from the last game."