Bob, I purchased your Instrument Rating study guide again to get up to speed on the part 91/135 changes. It has really helped highlight the differences, thanks.

One question that I do have is the minimum airborne equipment to fly IFR.

My understanding of navigation requirements are as follows:

(1) For private: permitted to have one aid in the air. However, if the destination requires an alternate and the aid is GNSS TSO 129, then the aircraft must also have a radio aid. In terms of private operations, practical operation means the aircraft must either have 1x TSO 129 & 1x radio air, or 1x TSO 145+.
(2) For transport: at least one RNAV / GNSS is always required (so 1x TSO 145+ & 1x radio aid or 2x TSO 145+).
Also must be able to survive the failure of an airborne aid and still have a means of conducting the IAP. So this either means duplication of the GNSS or aid.

My question is - would aerial work sit under category 1 or 2 please? I have checked CASR part 138 and cannot see any navigation stipulation for this category of work. I see the Definition of Air Transport in the CASA dictionarly Part 2 Para 3 as "a pax transport operation or a cargo transport operation conducted for hire or reward." So this sounds as though aerial work is a subset of Air Transport and so 2 would apply - is that correct please?

Airwork does not necessarily means pax. Airwork can be e.g. surveying, AG flying, etc where no pax are involved.
If any pax is involved, then that would be Part 135, otherwise it is Part 91.
Have a look at BT Irex extract. Part 138 is only mentioned under Part 91 section, but nothing mentioned under Part 135.
I wouldn't expect word AWK to be mentioned on IREX exam without specifying either flight with/without pax or Part 91/135.

NB: above is my personal opinion