Would that be legal in Australia? Aside from the fact that there isn't a great call for cattle mustering in the UK (actually, none as far as I know!), it would not be legal for a PPL to be employed by a cattle station.
The work may well be private (i.e. not public transport and outside the requirement to operate an AOC) but any earnings would classified as remuneration on a PPL.
The only way round that would be to be employed as say, an accountant and do the occassional flight as flying is not the primary reason for your recruitment. Being employed as a pilot means you are working commercially.
Cheers
Whirls