A significant number of trees are removed every year in Adelaide based on a perceived risk that is often greatly exaggerated. Please make sure that anyone providing you with advice on trees and/or risk management is qualified to do so.
If a tree worker immediately condemns your tree to be removed without talking over the reasoning behind the decision, ask yourself if they have something to gain financially from that advice.
A second opinion (and quote) from a qualified Arborist, may give you a totally different perspective. Once you have all the facts you can make up your own mind.