Ok, it looks like two votes to each possible explanation.
My thinking is its the speed brakes retracting as the pitch of the noise increases which gives the impression of an air gap closing as opposed to opening.
Would love to hear from an NZ A320 driver who would know straight away what normally happens at LOSGA while on the 23L STAR at NZAA.
All said and done, its a neat noise, sounds for all the world like the Jet is coming out of hyperspace or something and is unique to the A320 (we dont seem to get the shorter or longer versions into Auckland)
Thanks again.