The New Zealanders accuracy with their clearances at the collision area was spot on. Their exit plays were also completed with aplomb.
New Zealand led 17-15 at halftime and 20-18 after 57 minutes. A quick lineout and individual magic from fullback and All Blacks Sevens rep Vilimoni Koroi led to a 62nd minute try from Sullivan's replacement on the right wing, Tasman's Jamie Spowart.
Auckland first five-eighth Harry Plummer, who landed all five of his attempts at goal in tricky slippery conditions, provided the extras and that buffer proved handy during the final minutes. Outstanding Canterbury openside flanker and Baby Blacks captain Tom Christie was sin binned during the 77th minute but as was the case earlier on the Aussies were unable to capitalise.
"That was a real test," Christie said afterwards.
"That's like playing against your big brother ... no-one wants to lose. Credit to the Aussies ... they put up a hell of a fight and applied really good territorial pressure."
England will take on South Africa in the other semifinal.
Scorers:
New Zealand 27 (Harry Plummer, Devan Flanders, Jamie Spowart tries; Plummer 3 cons, 2 pens), Australia 18 (Fraser McReight, penalty try tries; Isaac Lucas 2 pens). HT: 17-15.