I would guess based on the shape of the badge that they are from the ANZAC’s? Likely taken on the Somme 1916, just my guess.

@... DarkDsurion I ruled out Gallipoli as I can’t recall there being any instances of gas being used during that campaign. Also the terrain appears different that the hills and cliffs you would find in Gallipoli. According to their history the ANZACs operated as mounted troops in Egypt after being pulled back from Gallipoli, the battles in the middle east were more fluid without the extensive use of trenches and pillboxes as you would see on the Western Front. The ANZACs were involved in the 1916 Battle of Pozières which took place in July as part of the Somme offensive, it was a primarily Austrailian/ANZAC effort, and the battlefield would have been somewhat dry due to summer weather.