Well, can't complain this time. Perhaps because instead of calling the technical help desk I was trying to reach the accounts dept, which probably made things a bit faster.
My call was promptly answered, and we managed to change my current 10Mbps/1Mbps/10GB plan to a 10Mbps/2Mbps/20GB plan.
I know the New Zealand traffic on this plan is now charged at 1:1 instead of 10:1 as before, but the allowance increase should be ok - we barely go over 10GB in a month.
The changeover is tomorrow - I already have the new IP address so it should be smooth...