There is an excellent dictionary and thesaurus built into Mac OS. It is used automatically by Mail and text edit. You can also use the dictionary and thesaurus by itself, it is located in your applicationa folder.
If you highlight a word in Mail, Text edit, Safari, and some other programs (hint, if you double click on a word it will select the whole word), and right click (hold down control, and then click) it will show a menu where you can select “Look Up in Dictionary. The dictionary will also show you the thesaurus entry for that word if there is one.
Right clicking on a misspelt word will also suggest the correct spelling.
If your dictionary does not like Australian spelling, you can change it by opening the spelling panel from Mail or text edit and selecting the Australia English dictionary. You can find the spelling panel under the edit menu, or by right clicking on any part of a document and choose spelling.