Hi - I have found a solution to update my Telecom NZ Samsung Galaxy S to what I think is an official Android 2.3.3 (Gingerbread), release XWJVB.
(1) For clear instructions to do a firmware update to 2.3.3 (XWJVB), see: How To Update Samsung Galaxy S I9000 to stable Gingerbread XWJVB 2.3.3 Firmware | Android Advices
(2) For clear instructions to do root the 2.3.3 (XWJVB) firmware, see: How to Root Galaxy S on Android 2.3.3 XWJVB Firmware | Android Advices
(3) There also appears to be a further update to a later version (from XWJVB to XWJVH). See: How To Update Samsung Galaxy S I9000 to stable Gingerbread XWJVH 2.3.3 Firmware | Android Advices
I have done steps 1 and 2 on my Galaxy S today successfully.
The two only additional instructions that I would add before anyone commences the update process is to
(a) read everything to the end and make sure you know what you have to do; and
(b) before commencing step (1) above, to note down all your TelecomNZ "APN" settings. To do this:
(1) Press the middle Home button
(2) Then press the left button
(3) Select "Settings" > "Wireless and Networks" > "Mobile networks" > "Access Point Names"
(4) For each entry you see (I had "TelecomData" and "TelecomDefault" on mine), select it to view the settings, and write down each setting you see. You will need all this to re-set your APNs after the update.
You don't have to "root" your phone (step 2) if you don't want to, but I did. I followed the instructions in steps (1) and (2) above, and my phone is working normally so far.
Hope this helps any other newbies like me out there.