Am I required to renew my passport after losing 20Kg?

I suspect you will be very much more conscious of your change in appearance than others are and I doubt even eye and nose surgery will have made you unrecognisable from your photograph (unless of course this was major reconstruction, say after a serious accident, rather than cosmetic).

Maybe ask one or two of those around you whether there is an 'adequate' likeness. I am aware that the cost of replacement is AUD250 and presume you would prefer not having to do so if not really necessary.

I recall that when I presented a photograph intended for a passport application to a responsible person to confirm it was a true likeness, before I could even explain why I was presenting the photograph, his question was "Who's this?". (He signed anyway and I never had any problems with it for the full ten year life of my passport).

There is a negligible chance that it will be an issue for you - the most you might suffer is a ticking off and suggestion you renew before your next trip. But since you are hardly likely ever to bump into the same official on different trips you might be prepared merely to accept a ticking off each time.

On the other hand, you really don't want complications that might cause you to miss an expensive flight and if you, and others, really believe you and the photograph are unlikely even to be taken for cousins then for your peace of mind the price to be paid may be worth your while.

Guidance from GOV.UK regarding photographic standards includes a section (at the very end) concerning plastic surgery. (I would expect requirements from other countries to be similar.)

12. Is the full fee required from a customer when he/she has undergone plastic surgery or suffered trauma to their face and the examiner cannot identify the passport holder from the photo in their passport?

Yes, when there is a permanent and significant change to an individual’s face such as they cannot be identified from the photograph on their passport, then they should apply for a passport at their own cost

I suggest the point to note there is that it is definitely not the case that a new photograph/passport is required after plastic surgery regardless of its extent.

To repeat that the guidance above is for UK (possibly for Australia is different but likely to be very similar since international guidelines are involved) but the guidance does require that at the time of a passport application the accompanying photographs are Taken within the last month. Your passport is quite likely due for renewal, due merely to the passage of time, in four years. Buying a new one now rather than waiting would in effect be to lose four year's use out of the ten years acquired. So you might consider the effective cost of renewing early as AUD100, rather than the full AUD250. The (UK) guidance also cautions:

As more countries install automatic border controls, anyone whose passport photograph does not meet the standard may be delayed by more thorough checks. It is in our customers’ best interests that we make sure they submit good quality photographs that comply with the standard, to prevent these potential delays.