"They all worked their hearts out,'' Southby said. ``We did take a little bit to adjust at the start but once we got used to it we knew it was going to be fast, furious and physical.
"We took it in our stride. It wasn't perfect by any stretch of the imagination but they nailed it when they needed to.''
New Zealand looked to have the final under control when they led 46-38 with just under nine minutes to go in the fourth quarter.
But a change in the Australian shooting circle during an injury time-out saw momentum change with the combination of goal attack Gretel Tippett and Cody Lange adding a spark to their frontline.
It took some time for the New Zealand defensive combination of Zoe Walker and Phoenix Karaka to adjust to the changes but the Kiwi combination, who worked well during the match, picked up some key ball late in the match.
Wing defence Stacey Peeters was also strong for the under-21 side with her work around the circle and her support play through court.
New Zealand showed nerves early on and trailed by two goals mid-way through the first quarter as Australia used their strong links to tall shooter Kristina Brice.
But Walker and Karaka plucked some impressive intercepts late in the quarter which were converted at the other end of the court by Malia Paseka and Te Paea Selby-Rickit as New Zealand finished the spell with confidence, heading into the first break with a 15-11 lead.
Australia made changes in their defence line in the second quarter but it did little to stem the flow of goals as New Zealand maintained a six-goal lead at half-time.
New Zealand coach Janine Southby switched Paseka to goal attack and Selby-Rickit to shooter heading into the third quarter and the duo took time to find their rhythm as Australia surged to within two goals mid-way through the spell.
A big intercept from Walker sparked the Kiwis late in the quarter and it was New Zealand who went into the deciding 15 minutes with a crucial five-goal buffer, leading 41-36.
Mid-way through the fourth quarter, Australia opted for more changes with Tippett making an impact off the bench at goal attack as the green-and-golds again closed the gap to just three goals.
A vital rebound from Selby-Rickit late in the game gave New Zealand a four-goal advantage with two minutes remaining and from there the Kiwis held on for a thrilling win.
Earlier, Jamaica won the bronze medal with a 52-33 win over England.