Vodafone released its quarterly results a couple of days ago which lets us calculate this:
Vodafone's customers on 31/3/07: 2,244,000
Telecom's mobile customers on 31/3/07: 1,936,000
Total mobile customers: 4,180,000
NZ population on 31 March: 4,177,000
Therefore total penetration = 100.07%...
Way to go!