Tokyo - Jordie Barrett's unruffled performance as a makeshift All Blacks flyhalf won the praise of coach Steve Hansen - and put him one up on his big brother, two-time World Player of the Year Beauden.

The Barrett family accounts for nearly one-tenth of New Zealand's 31-man Rugby World Cup squad, with playmaker Beauden the eldest brother and the senior All Black compared to Scott, a lock, and utility back Jordie.

But Jordie, youngest of the brothers, earned bragging rights when he took a personal haul of 21 points in New Zealand's 71-9 win over Namibia - bettering Beauden's World Cup high of 16, scored against the same opposition in 2015.

A try and eight-from-nine kicking in the second half - after he missed his first two conversions in the first - left the 22-year-old "elated", according to Hansen.

"I think he's pretty excited and happy with how he played, how he did the job that Ian (Foster, the assistant coach) and I wanted him to do, and the team needed him to do," Hansen said.

Jordie has a long way to go to improve on the exploits of his big brother Beauden, who scored a try in the final as New Zealand won the 2015 World Cup and was World Player of the Year two years running.

When told he'd beaten Beauden's personal haul in a World Cup game, Jordie said "that's nice to know" but stressed he wasn't out to get one over on his brother.

"That's one thing I don't go there and play for. I'm just happy I found my rhythm off the tee after missing the first two," he said.

"It's one thing I've been working at the last couple of months, my goal-kicking, so it's nice it came off."

Jordie's pride in his performance is understandable. He has told how he was often left in tears when the Barretts' competitive instincts took over during games on the family dairy farm in Taranaki.

After playing on the right wing against Canada, he was drafted in at flyhalf at the last minute for the Namibia game, after being given just two days to prepare.

Given he hadn't started at flyhalf since university, it was an assured, all-round performance from the youngest Barrett, whose cross-field kick set up the opening try.

He stood up to a physical test from the Namibians, made a storming break to create Anton Leinert-Brown's second try and capped his afternoon by diving over for his own score five minutes from time.

Jordie also had to steer the defending champions through some sticky moments when they were just 10-9 up against a willing Namibia team after half-an-hour.

"I loved it, loved every minute even in that first 30 when we were up against it, as a good challenge for me mentally to stay in focus, be where my feet are," he said.

"I enjoyed today, felt like I could grow into the game as it went on."

Hansen, who will step down after the World Cup, said he wasn't sure if Jordie would play flyhalf again.

But the All Blacks are on alert for all eventualities after 2011, when injuries to Dan Carter, Colin Slade and Aaron Cruden handed fourth-choice Stephen Donald kicking duties in the final.

"We all know he's not a first five-eighth, and I don't know how many Tests he'll play there, because after this tournament I'm not going to be in charge but it's certainly not his number-one position," Hansen said.

"So to be able to come out and play like he did, he should be really, really excited and fulfilled and proud."