The obvious answer is what you need / want in retirement and how long you will live for, however, according to the Association of Superannuation Funds of Australia, the following per annum is required:

Modest Retirement:
Single     $23,489
Couple    $33,784

This would be better than the aged pension but still only the basics would be available

Comfortable Life Style:
Single $42,597
Coupe $58,326

This would allow for a broad range of leisure and recreational activities.  As well as access to private health insurance; reasonable car; the occasional overseas holiday.