I believe the possibility exists that depending on how your ISP or local phone provider hardware firewalls or partitions out their data infrastructure that you could have the same IP as say, someone who lives in your same Apartments, building, or neighborhood.
Many cell phone providers segment their 2G, 3G, and now 4G data from one particular Cellular tower cellular switch traffic into one stream which has to be back-hauled to a POP (Point of Presence) Central Office phone company location through one RF wireless WiMAX type circuit point to point circuit, which could give anyone with a cell phone oriented Internet data connection the same IP to the rest of the Internet.
This I know about because I am a Wireless Broadband Field Service Engineer, working for a company that does just this sort of cellular tower WiFi back-haul!! If you are using a wireless WiFI router in your apartment/house/shack and someone unbeknownst jacks into your WiFi is another possibility.
Also, if you are coming into the Internet from a business or corporate network that is firewalled and has one point of presence to the public Internet through a Internet Appliance or Hardware Firewall device then anyone from that business/location will all have the same IP to the public Internet. Best example of this (other than you frakkin your employer and playing SGW games whilst you are SUPPOSED TO BE WORKING - HAHA we have all done this!! don't lie...) is if you are at an Internet Cafe or coffee house, Starbucks, etc. using a shared WiFI broadband that could all appear as the same IP to the public Internet from that one coffee shop or restuarant.
Anyway, if you are using a little Cell-phone "dongle" or Broadband modem card from your laptop/PC or data "tethering" your Cell phone to your Laptop or PC and using your Cell/Smart phone like a broadband modem then anyone on that same local cellular tower you are getting your signal from can potentially have the same IP address!!
Hope this helps... Now Affy must go rest, my brain is farting and having a meltdown...
What you explained makes a lot of sense 
Fair bit of distance between Brisbane and Newcastle to be on the same cell tower exchange (which is where I'm guessing Alucard and yourself are?).
