To be certain:
1. Consult your owner's manual: This is the definitive source. It will show you the location and the correct size and type of sump plug.
2. Look underneath the car: Locate the oil pan (the bottom of the engine). The sump plug will be on the underside of the oil pan, likely towards the front but possibly slightly offset. Be careful when looking underneath. Use jack stands and never work under a vehicle supported only by a jack.
3. Check online forums: Search for "2005 Holden Astra oil change" plus your specific engine size. Other Astra owners might have pictures or descriptions that match your vehicle.
Don't assume it's a Torx just because you think it might be. Verify before you start working on it to avoid stripping the plug. Having the correct tool ready will save you time and frustration.