VT Commodore 6x9 Speaker install
Sure, it's OK as long as the new 6X9's are the same impedance as the door speakers.
Unless there are already wires and cutouts for rear shelf speakers, the installation will be pretty involved. And while I'm not sure of the Holden product, it's sister versions, Chevy Lumina and Omega, does not have such cutouts or wiring.
Wires going into the door interior for window controls, locks, and speakers will all be in a bundle and go from the vehicle frame into the door through a flexible rubber connector with grommets on both sides. You'll need to run your wires from the disconnected door speakers back through the same grommets to avoid pinching and damaging them. After you get them back into the vehicle interior, you can run them under the trim or carpet to the trunk area under the rear shelf. If you have never done such work before, I recommend you get someone who has to help you. You'll need to make an appropriately sized cutout to mount the 6X9's. And depending on the material, you may need some kind of reinforcement and/or spacers. Also, look out for anything underneath the trunk lid that may come into contact with the speakers. There's usually 2 metal rods than span the width of the trunk and run almost directly under the shelf. They will have to clear the speakers in order for the trunk to open and close properly.
It's more work than a simple speaker exchange with a good pair of aftermarket speakers. And the benefit is minimal. A really good set of 5 1/4, 6 1/2, or 6 3/4 Alpines, Bostons, or Infinitys will sound better than a mediocre set of 6X9's any day and you don't have to cut everything up to install them in most door openings.
Hope this helps.