Pretty sure the front roll bars are either 22mm or 23mm, with the 23mm ones being hollow. My 156 V6 bar is 22mm, I think the 23mm ones are usually on 147's. To be sure you'll need to measure yours. GTA and GT 3.2 are different again.
It's a fairly straight forward DIY job - if you can get the section of exhaust that runs under the subframe off then you're half way there. You'll need a 12mm drill to drill out the rivets that hold the roll bar bush clamps together (unless the bushes have already been replaced).
You are providing erroneous information, certainly with respect to the UK market.
The standard ARB on the 147/156/GT is 22mm circumference, Alfa Romeo part number 51754198.
The uprated ARB with 23mm circumference was fitted to the 147 GTA, 156 GTA, and GT V6 only. Alfa Romeo part number 51754199. The 1mm increase is modest, but it does provide more assured handling particularly with the JTD models, which is why I suggested it, and it is why some Alfa owners have done this modest but effective mod.
EPER front suspension for the 147:
EPER front suspension for the 156:
EPER front suspension for the GT:
The New Zealand market may be different, as Alfa Romeo did have a habit of altering some of the specs of these models for specific markets. But the OP lives in the UK, and this is what he needs to know.