Townsville is on the Bruce Highway, 4 hours from Cairns and 3 hours from Airlie Beach (Whitsundays). You can bring your car to the island on Magnetic Island Ferries car barge service. If you prefer, you can park at the barge terminal or at the Sealink ferry terminal.
This is the cheapest transport to the island for both passengers and vehicles. They operate multiple return services daily between Townsville and Magnetic Island, 7 days a week. They also provide very secure free parking which is locked up at night.