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https://support.mozilla.org/en-US/kb/how-do-i-use-firefox-spell-checker

Karen, here are several 3rd party dictionary plugins for Web Browsers. You need to evaluate them to determine if they meet your requirements. If they don't we'll need to spend more time editing them or making a custom dictionary. Or you may want to talk to the publishers of the Macquarie Dictionary and see if they have a web browser plugin.

Two for Firefox, the first looks better maintained:

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Microsoft Edge on Windows 10 has built in spell checking. IE 11 has spell checking built in in Windows 8. You may need to install a language pack for Australian English though:

• http://windows.microsoft.com/en-AU/windows/language-packs#lptabs=win8

IE 11 on Windows 7 apparently has a spell checker built in, although some people have complained about it. You'll need to test this scenario to see if we need to provide more support here.

•  http://www.askvg.com/how-to-disable-built-in-spell-checker-feature-in-internet-explorer-10-ie10/

I don't have any information about Safari or Macs. They are currently listed as a supported platform, so we either need to a) test them and find plugins / settings as appropriate or b) not support them!