If I leave the UK to Australia on April 1st 2012, and return April 30th 2013 (therefore clearly leaving for a full tax year) and do not return at all during that year, I would then become non-domiciled. Correct?
Incorrect. You are confusing residence and domicile. Domicile is far more "fixed" than residence. It depends on your father's domicile, the location of your family, the location of your assets, the location of any business interests, under which country's law your will is made, your long-term intentions.
In simple terms domicile is about where your roots are. In early years a person's domicile will usually match that of their parents. It is possible to "re-root", but domicile does not change as often as residence might change. Even if someone lives abroad for 30 years they may still remain UK domiciled. It depends on a lot of things.