One photo is titled 18 pounders....
Douglas,
The 18-pdr was mentioned at the end of the article I posted: "Inadequate artillery pieces and tactics were replaced after the Second Boer War. A new quick-firing 18-pounder, with gun shields, sights, and hydraulic/spring buffer was introduced. It was originally employed in two-gun sections until battery firing was again shown to be more effective and efficient."
I can't tell by the picture, but just going by the date and description of the first photo, along with the fact that the material is from the 'Australian National Archives' would suggest that (as Max said) it's the British "Ordnance QF 18-pounder" that was introduced in 1904.
http://en.wikipedia.org/wiki/Ordnance_QF_18_pounder