If you want a correct answer just go to the VIC Roads regulations.
They publish on the net, VIC roads "Guide to Modification to Motor Vehicles" which covers those items that can and cannot be done without a specific approval. In that document they apply the national rules for tyre and wheel modifications and this is contained in this VSB 14 document which they have clearly referenced.....
https://infrastructure.gov.au/roads..._LS_Tyres_Suspension_Steering_Nov_2015_v4.pdf
This contains all you need to know.
You won't need to put 10.5 wide wheels on your car. There are hundreds of choices in 20 inch in 8.5, 9 , 9.5 and 10 inch.
Some basic rules you will need to comply with.........
1. Retaining speedo accuracy which is affected by tyre diameter (avoiding the speedo significantly under reading the true speed because of too large a tyre diameter compared with stock. No one cares if the speedo reading exceeds the actual speed.)
2. Tyres and wheels must clear suspension and body work. (You may need to roll the rear guards on your car depending on the wheel width, wheel offset and tyre choice. If the tyres are contacting suspension components on the inside front your wheel offset is wrong or the tyre choice is wrong or both are wrong and you will have to change them.)
3. The tyres and wheels must sit within the guards of the vehicle and thus not protrude beyond the face of the guards.
4. The tyres you choose must have a load rating not less than the lowest load rating
"listed on the tyre
placard of the vehicle or equivalent variant of that model vehicle"
5. The tyre size fitted must conform to the manufacturer limits on rim width for that tyre size. Manufacturers stipulate the lowest and highest rim width approved for each of the tyre sizes they produce.
If you are fitting different size tyres front and rear because of wheel widths
you need to keep the F and R tyre diameters as close as possible. Large variations will cause the stability control system in your car to be activated.
If you stick with the common staggered configuration of wheel and tyre upgrade (as below) you won't go wrong and you will comply
but you will have an issue with tyre load ratings when upgrading from 18 inch to 20 inch tyres (which many ignore). If you go for 19 inch wheels and tyres you won't have that load rating issue because these were fitted stock to the VE SSVs. You won't have an issue either if you had the optional 20 inch Holden wheels fitted when the car was delivered because the tyre placard is changed to accommodate the 245 35 20 tyres that were fitted to the wheels..
Front 20 x 8.5 or 20 x 9 inch wheel Tyre 245 35 20
Rear 20 x 9.5 or 20 x 10 inch wheel Tyre 275 30 20 or 285 30 20 (subject to rear wheel offset),
A 255 35 20 front tyre (with a lower offset wheel than the stock 48 mm) with 285 30 20 rear tyre (subject to rear wheel offset) will also work.
If you are going to fit a 9, 9.5 or 10 inch REAR wheel with a 275 wide tyre, a wheel offset that is lower than 45 mm will require the rear guards on your car to be rolled to prevent tyre contact under full suspension movement. You will find a lot of rear wheels in the 35 - 45 offset range. If you don't want to roll the rear guards then look for rear wheels with a 45 or higher offset. Your stock 8 inch wheels are 48 mm offset. Wider REAR stock Commodore or wider REAR optional Commodore(in 9 inch or more) and wider REAR HSV wheels are typically 57 offset to avoid any rear guard clearance issues.
