Grumpy, I think that Jaffa is already on the right page here, as far as the correct tool for the job goes.
'Roll pin punches' are a parallel pin punch, with a specially machined tip. Rather than being flat, the centre is raised, in a ball or truncated cone shape, to centre the punch in the hollow pin, and prevent sideways slippage. They're not a very common tool, but they are available.
Jaffa, Snap-On Tools do sell single Imperial size roll pin punches. But as you may know, Snap-On ain't cheap, though their quality is good. Their Oz online shop quotes $22.85 for the 3/8" size, Product Code PPR12. No idea what they want for delivery. There will be a franchisee who services motor trade workshops in your area, though.
A possible alternative is the T&E Tools range, which is probably what Repco is offering. No idea about the quality of these. A Google will find some local retailers; one online shop [sostools] I found offers both sets and single punches for good prices.
If you know someone who has a lathe, as grumpy says, these punches aren't hard to make. The 'silver steel' he mentions is a precision ground hardenable alloy steel rod, available from industrial tool and bearing suppliers. The Yanks call it 'drill rod'.
Another cheap, ready source of decent tool steel is old car shocker/strut piston rods. These are made from 'AISI O1' steel, and harden well if heated to bright red and quenched in oil [this avoids excessive hardness] after machining. The hard chrome plating can be ground off, or machined off with carbide tooling.
