I don't know whether this still applies, but employees paid above the award were exempt from award conditions several years ago. This could work to the employer's advantage at times.
From a brief scan of SEEK, it seems 'assistants' are worth 30-70k in Melbourne.....suppose it depends on
- how big your business is
- your (RedCat) position and value to the company
- whether she is more an office, personal, or executive assistant.
I think it helps to remember too, that no one is indispensible....and one day she may meet a guy who drags her off to Sydney or overseas or sprogsville.
I think it also pays for small business to model the practices of larger business, and in this respect, that means ensuring more than one person can perform her duties. Consider it risk mitigation.