He clouted Steyn for three fours and a six within five deliveries before the South African pace bowler eventually dismissed him.
"If he wanted to be more courageous and brave he would have been 30 off 90 and watched his captain get to 100 rather than score 31 and walk off the pitch and say 'I've done my job'. That's not your job.
"He gives his all but I would have tried to hang out there and even get 10 off 50 balls."
Steyn also analysed Wagner's recent short-pitched bowling methodology.
"He's at that pace where... you always feel there's a chance you can take him on, but he is a lot skiddier than you think and he has good skill.
"He gets his bouncer really high. He doesn't bowl chest high which is an easy height. He hits the head. He makes it difficult for the batters to play.
"There's always the risk of when you hit the bouncer you might hit it in the air with the chance of getting caught."
Steyn suggested New Zealand had little chance of survival.
"The longer the sun bakes the wicket and we get our bats into the cracks and widen them up a bit, it's going to be more difficult to bat," he said.
Play starts at 8pm tonight (New Zealand time). Steyn is listed to bat at No.10. Williamson's captaincy modus operandi will undergo scrutiny as to how he uses Wagner during that period.