Pick your dates wisely. August is typically the cheapest month for a rental in Coffs Harbour. Watch out for the September school holidays, when prices often hit their yearly high.
Compare airport vs. city pickup locations. A quick taxi ride to an office in town could save you a decent chunk of change on airport surcharges.
Look at weekly rates if you're staying 5 days or more, as the daily price often drops. Also, picking up on a Tuesday is usually cheaper than a Friday.
Skip the prepaid fuel option and fill the tank yourself before returning the car. For a bargain, fuel up in Toormina, where prices are often lower than in the city centre.