2008 Holden Astra only driving in Reverse

You might be lucky and simply have a low transmission fluid level thanks to a leak, or perhaps even a selector that is not properly connected to the transmission.

But the fact that the transmission works in Reverse but not Drive suggests something internally wrong. You could be looking at a broken torque converter, failed clutch or band, a faulty pump, blocked valve body or even a broken gear set. A transmission specialist is going to be your best bet here.