Without looking at the Vicroads documentation and going from memory, modification to suspension (including arms etc., but not cross members) is not allowed.
That being said, if you are only drilling a single hole in an area that doesn't have any effect on strength and loads passing through the suspension arm, such as the semi-trailing arm, then I don't see why this would be an issue.
Vicroads and VASS Engineers (if in VIC) are concerned if you weld suspension components also. That being said, if you are a certified welder, you are ok. Basically, if you know what you are doing, it should be fine...
Where would you be drilling the hole on the top surface?
1972 Datsun 1600, S14 SR20DET Engineered (204rwkW @ 17psi.)