It used to be 10 years, might be 7 now though.
However it only applies while the supplier is still in business (I was looking into whether Dad should get an ED Falcon or Fairmont, the main difference from his POV was cruise-control, but even though they were only 5yo cars & there was a 10-year law on parts we couldn't get the cruise-control kit because the supplier had closed-up shop) ... and a lot of suppliers for the Australian car industry are going out of business.
That's not correct, Holden is responsible for supplying parts for 'a reasonable amount of time' after a model has been released. Internally it is between 5-10 years depending on the product type. They have a massive warehouse in Dandenong that holds all of their spare parts.
If there is a supplier that can no longer supply parts, it is upto GMH to either do an 'all time buy' (purchase more than enough stock in one go to outlast demand) or to re-source the product to another supplier. There have been times where an ATB was completed but due to a spike in demand, stock runs out before the minimum time that they need to supply, which requires them to resource a part that is over 10-years old.
Also, whenever it is decided that a part is at the end of its life and the tooling that is used to produce it is ready to be scrapped, Rare Spares are offered the opportunity to take the tools and continue production. If they decline, then the tools are destroyed, but a lot of the time they take the tools and store them, waiting for demand to pick up again so that they can produce the parts for the market.
At the end of the day it is Holden's responsibility to ensure ongoing parts availability, regardless of the state of its supplier base, as per ACCC regulations.
I know this because I used to work in the aftersales purchasing department at GMH, which was responsible for ensuring continued supply of parts.