Burke and Wills tree page on the Time Travellers website, and done some further research in Trove, I have reviewed and changed some of the dates of the photos there. Dave Phoenix of the Burke & Wills Web helpfully put me onto two further images of the tree at the National Library of Australia website (currently waiting for permission to publish them). These have been mislabelled as being at Royal Park, which is how I came to miss them. The NLA photos have gas lamp standards in them, which had dating implications - not before 1883.
Alex Bragiola pointed out that the houses in the background were built in 1883. Also that one of the photos was taken looking at Queen's Park, and the picket fence had been replaced by a wire fence in 1915. The picket fence was built by Council in 1885. It is great to have some help in refining the dates on these photos, so thank you Dave and Alex.