The climate zone system is designed to idenitify the minimum expected temperature of an area.
Some plants require very specialised environments while others are more tolerant.
Special attention should always be paid to cold areas, however, as many plants are not frost hardy and will simply keel over and die if a frost gets to their roots. Most plants in the Gardens OnLine database will grow in average Australian conditions, but trying to grow certain plants in the wrong zone is just not worth the effort.