In front of a vocal home crowd, Belgium defended hard but goals from captain Anita Punt and Gemma Flynn sealed the result for the Black Sticks.
New Zealand finished on top of pool B with four wins from four games as well as an unbelievable 21 goals scored for and none conceded.
The Black Sticks are now confirmed to play Japan in the sudden death quarterfinal on Wednesday at 6.30am NZ time.
Flynn, who has scored seven goals at the tournament so far, said it was good to get the result and now the focus would shift to the quarterfinals.
''It's good to come through the pool games unbeaten and with a clean sheet. I'm really proud of our defence and the way we are playing for each other,'' she said.
There were a few things to be worked on.
''But it's good to take away the win and now our focus really turns to Japan.
''Japan can play a similar way to how Belgium did today and they defend hard, so we need to counteract that. We know what we need to do to get it done.''
The Black Sticks had all the early running and possession but Belgium was buoyed by its home crowd and defended desperately.
Punt opened the scoring in the 12th minute when she smashed home her own rebound from a penalty corner flick.
Both sides had their chances throughout the second quarter but could not find the back of the net, and New Zealand carried a one goal advantage into halftime.
Flynn continued her hot scoring streak just three minutes into the second half, delivering a punishing blow from the top of the circle to give the Kiwis a two goal buffer.
The Black Sticks had the majority of possession throughout the rest of the game but Belgium pushed its players behind the ball and the Kiwis could not add to the score sheet.
The Black Sticks' match on Wednesday will screen live on SKY Sport.
World League
The scores
New Zealand 2
Anita Punt, Gemma Flynn
Belgium 0
Halftime: 1-0
