The penalty count was 12-7 against the All Blacks, and de Groot's card was upgraded from yellow to red in a game where the All Blacks were dominant in all other areas.
It was a chance for lesser experienced members of the squad to make an impression, and wing Leicester Fainga'anuku and halfback Cam Roigard led the way with strong performances, Roigard scoring twice and Fainga'anuku scoring one and having a hand in several others.
Fainga'anuku, Caleb Clarke and Anton Lienert-Brown set the standard from the start, Fainga'anuku especially. Having proved a handful with the ball in hand, he fielded a cross-kick and fed the ball inside for Roigard to open the scoring after 90 seconds.
Namibia pressured the All Blacks' line but turned the ball over just short, and the All Blacks cleared out downfield, fullback Beauden Barrett kicking the ball to the goalline. First five-eighths Damian McKenzie, in a 26-point performance with two tries and eight conversions, chased hard but grounded the ball just short. From the five-metre scrum, the All Blacks pushed Namibia off the ball, and No8 Ardie Savea cleared to Roigard, who cork screwed over for the second try, his second in the sixth minute.
There were three consecutive lineout drives, two resulting from penalties, and hooker Samisoni Taukei'aho who crossed. But, it was wiped out when prop Ofa Tu'ungafasi and lock Sam Whitelock blocked Namibia from getting to lock Brodie Retallick.
The All Blacks gained a fortunate penalty when losing the ball at the breakdown. They kicked to the 22m area and worked a move with flanker Dalton Papali'i, who fed Fainga'anuku running an inside-out angle to bullock his way across for a try after losing then regaining the ball in the tackle.
Namibia's first five-eighths, Tiaan Swanepoel, who landed their first points with a penalty goal, kicked a goal line dropout into touch, giving the All Blacks a five-metre scrum in front of the posts, but had a let-off when the pack went early and were penalised.
Lienert-Brown showed his skill when receiving the ball on the right flank. He put a grubber kick past his marker, toed it on and past defenders and had time to set himself for the ball rolling over the goal line to score.
Swanepoel kicked the restart dead, and from the midfield scrum, Roigard ran through a gap and reverse-passed to McKenzie, who scored in the 38th minute.
Into the second half, the All Blacks substituted their props in the 47th minute, and from the first lineout, a massive maul rumbled towards the goal line. From it, Roigard worked the line where substitute prop de Groot scored a try within a minute of taking the field.
A scrum foundation on halfway provided impetus after a break from Savea. Barrett stepped around a defender and set Fainga'anuku for the line. Quick clearance when he was caught saw Barrett from stand-off fed Papali'i into the gap to score beneath the crossbar.
Moments later, the lineout maul exposed the Namibia defences as Roigard tidied, ran and passed for second five-eighths David Havili to score.
Namibia had their best chance to score a try early in the fourth quarter and threw almost the whole team into a maul, but skipper and centre Johan Deysel dropped the ball, allowing the All Blacks to clear.
Substitute first five-eighths Richie Mo'unga made an early mark with a kick through towards the line, where Clarke patiently waited for the ball before clutching it and scoring.
De Groot was yellow carded for head contact in a tackle in the 71st minute.
The play became unstructured as New Zealand attempted to sort out playing with 14 men. When they regained their flow, Fainga'anuku made ground down his sideline before in-passing to replacement centre Rieko Ioane, who scored to round a record margin against Namibia.
New Zealand 71 (Cam Roigard 2, Damian McKenzie 2, Leicester Fainga'anuku, Anton Lienert-Brown, Ethan de Groot, Dalton Papali'i, David Havili, Caleb Clarke, Rieko Ioane tries; McKenzie 8 con) Namibia 3 (Tiaan Swanepoel pen). HT: 38-3