Importantly, mould cannot grow in a dry and well ventilated area. You need to remove moisture in the air and provide natural light to slow any mould growth. This can be difficult for most sheds, especially with sunlight. One thing to look at is a decent dehumidifier machine; a local Sunshine Coast company http://www.dampsolutions.com.au specialises in these and have a huge range of items to help with this. A good one can be somewhat expensive however, if the missus puts up a fight about it, let her know it also works great at drying the washing in the same area. A good compromise to make sometimes! If you don’t want a unit in the garage, an inexpensive, small, portable unit running in the car would be worth a look at, just be sure to check the water tray each week so it doesn't overflow. Also make sure it sits on a solid, flat surface and has a separate electrical cut out, as I'm sure we would all pick mould over a fire any-time!