Smail finished at seven-under-par after a double bogey at the last, but still won by two strokes over compatriot Alker, defending champion Michael Campbell (66), Australian Nathan Gatehouse (68) and Englishman Roger Chapman (69).
With Alker picking up two strokes through six holes with birdies at the first and fifth holes, Smail was level with his countryman after bogeying the fourth, but an eagle at six saw him draw clear again.
Alker twice got back within a stroke of Smail early in the back nine with birdies at 10 and 12 - as well as a bogey at 11 - after the leader had carded five pars in a row. However, a Smail birdie at 12 put him two in front again.
With Smail only parring holes 13-17, Alker, perhaps pushing to close the gap, made costly bogeys at the 17th and 18th holes.
After almost missing the cut on Friday, Campbell got within two strokes of Smail after 16 holes with five birdies, but a double bogey at the 17th ended his chances of successfully defending. He recovered with a birdie at 18 for a share of second.
Gatehouse had matching front and back nines, collecting birdie pairs before adding a later bogey each time, while Chapman bounced back from an ordinary start for the second day running.
His front nine contained three birdies, three bogeys and a double bogey, but three birdies in four holes from the 11th to the 14th holes allowed him to recover to a sub-par round.
Click here for Full-Field scores from New Zealand!