The following definition of the Australian-English expression cut-lunch commando is from The Concise New Partridge Dictionary of Slang and Unconventional English (Abingdon (Oxfordshire): Routledge, 2015), edited by Tom Dalzell and Terry Victor [2^nd edition, page 216, column 1]:

cut-lunch commando noun a soldier who does not see active service, especially a reservist. A contemptuous term implying that they get a prepared lunch ¹ rather than army rations. AUSTRALIA, 1952

¹ In Australian English, the noun cut lunch designates a packed lunch, typically consisting of sandwiches.

From The Argus (Melbourne, Victoria, Australia) of Thursday 7^th December 1944 [No. 30,664, page 9, column 1], the following photograph and caption illustrate the meaning and origin of the expression cut-lunch commando:

THE ARMY PLAYS A JOKE! The Army is reputed to have no sense of humour, but just look at this photograph and then say that this isn’t one of the best pieces of service humour ever perpetrated. Everyone, including the General Staff, must know that frontline soldiers call their base headquarters opposite numbers Cut Lunch Commandos. This photograph is of the dignified main entrance to a lines of communication headquarters, somewhere in Australia.

The earliest occurrences of the expression cut-lunch commando that I have found are as follows, in chronological order:

1-: From the column Between Ourselves, published in The Australian Woman’s Mirror (Sydney, New South Wales, Australia) of Tuesday 12^th October 1943 [Vol. 19, No. 46, page 9, column 2]:

“Dorf”: These unofficially given unit titles! Himself, a soldier of the last war, convinced by several Army doctors that his front-line days are over, now working on a headquarters clerical staff, cheerfully alludes to himself and his companions as Cut-lunch Commandos!

2-: From 50-50: Letterettes In Fifty Words, published in the Daily Telegraph and Daily News (Sydney, New South Wales, Australia) of Saturday 18^th December 1943 [Vol. 8, No. 233, page 13, column 3]:

Rats! Wynyard Ramp may have a few rats, but some of the Army camps have their issue. Last week our hut had settled down to sleep when there was a commotion in a corner and Bluey cried out: “Cripes, a bloomin’ rat started gnawin’ me ’ead!” The hut again quietened, when a small voice plaintively inquired, “Hey, Bluey? Was the rat tryin’ to get in or out?”—“Cut-lunch Commando.”

3-: From Knocking the Army, by Frank Browne, ‘Ex-VX’, published in The Ovens and Murray Advertiser (Beechworth, Victoria, Australia) of Wednesday 1^st March 1944 [No. 11,311, page 6, column 4]:

Sarcastic references to alleged Army inefficiency seems to be an all too popular sport these days […].
Periodically, some financial expert whose knowledge of accountancy is limited to drawing his pay on Friday, pushing it over bars and into poker machines from Friday to Monday, and borrowing his fare to work again on Monday—launches a diatribe against Army finance. He claims that red tape is holding up Diggers’ allotments, and that soldiers are being robbed by either inefficient or dishonest book-keeping methods.
[…]
Next best “Aunt Sally” ² to the Finance Branch seems to be the Canteen Service. This service, which has converted the average soldier’s life into a paradise compared to previous wars, is continually being accused of making excess profits out of soldiers. The peculiar fact about the charges is that you never hear them being made by fighting soldiers—or those who are buried in some remote part where they are entirely dependent on Canteen stocks for the little amenities which go towards making life happy. Yet you’d expect that if the Canteen Service was exploiting anybody, it would be these fellows who have nowhere else to buy their things.
No. The big moans come from the base wallahs, who from time immemorial have always nourished the idea that the fighting soldier was a rough uncouth fellow for whom even the second best was far too good. So that when these fellows raise a moan about paying one and sixpence for a bottle of beer, although they know that by so doing they are helping the bloke in Moresby or Darwin to get a bottle at the same price, they are running true to form.
The fact is, these fellows are quite ready to let the chap in remote areas be slugged for the pre-war Darwin beer price of 3/6 per bottle. It’s not their worry. The Canteens Service is able to provide it at one and nine a bottle only because it charges that price all round.
But the cut-lunch Commandos don’t like that. So they squeal to the papers and in many cases they get a hearing.

² The expression Aunt Sally designates someone or something that is seen or set up as an easy target for criticism.