From the model I came to conclusion that the aircraft likely was:
- British due to it being spotted Australia
- Built in 30s as opposed to 40s due to boxy fuselage shape
- Civilian or transport plane - due to contouring in upper fuselage
- Trimotor due to similarity between engine and nose shapes
With that I looked through Wikipedia's list of 1930s British airliners and found:
Features that match:
- General fuselage excluding contouring
- The steep vertical profile of the windscreen (somewhat uncommon in other planes)
- Engine shape and placement including nose mounted engine being the same but larger than wing mounted engines
- Split tail horizontal stabilizer
Features that don't match:
- Contouring on top of fuselage - probably mistaken impression by artist delimiting passenger compartment from fuselage accentuated by horizontal paint stripe. I personally thought Ju-52 had that feature but was also mistaken.
- Wing shape is different - the model has more forward swept wings
- The leading edge of tail horizontal stabilizer is angled instead of perpendicular to fuselage.
With that I was still skeptical about my findings, given that according to Wikipedia there were only 17 such planes without any mention in text connecting them to Australia. That is until I noticed that the picture there was titled "Spartan Cruiser over Melbourne, 1934"
Seeing that I searched the plane number G-ACDW and found its history that places it in Australia:
In the period October to December 1933 a Cruiser II G-ACDW (c/n 3 – Faithful City) was flown by Patrick Windsor Lynch-Blosse, chief pilot of Spartan Airlines, from England to Australia, leaving Lympne on 10 October and arriving in Brisbane, QLD on 29 October after 110 hours flying time. Lynch-Blosse had previously flown for a short period in New Guinea, and at another stage flew in command of the Avro X Southern Sun in Queensland in 1930.
On 30 October the aircraft was flown to Sydney, NSW, later through Victoria, up the west coast to Derby, WA having circled the continent. It returned to the United Kingdom, arriving on 26 December 1933. This flight was a long-distance charter and was claimed by the owners to be the longest private charter of its type undertaken.
That means that your artist might have seen the plane in person some time between 29 October 1930 to 26 December 1933 or from pictures some time after that, considering that the longest private charter flight ever might have ended up in local press.