2005 Holden Astra not cranking

You could have a fault with the car’s wiring. The starter-motor takes its power from the fat, red and black leads that attach to the battery, but it also needs a low-voltage signal from the ignition key that tells it to do its thing. If there’s any sort of loose or missing wire, you might not be making the necessary electrical connections. You could also find that there’s an on-board security system that is blocking the starter-motor from turning. You could even find that the ignition key has a flat battery and is not unlocking the car’s on-board computer.

You say that the battery has voltage, but I wonder how you checked this. You can often put a multimeter across the battery terminals and see 12 volts, but this can be what’s called a surface charge that’s strong enough to light up the dashboard, but then disappears when you put the battery under load (by trying to start the car). Attach the multimeter and check the voltage while actually trying to start the engine. You may find you have a dead battery after all.