I was at the Mungo hot springs recently. The upper one which is on the True Left of the Mungo River approximately 60metres above the confluence with Brunswick Creek appears to be full of gravel & needs digging out. It is easy to detect because of the strong localised sulphur smell.
The lower spring is also on the True Left of the Mungo but at least 300m below the confluence with the Brunswick. It currently is in the form of a shallow hot water stream (nice bath temperature) which could easily be channelled into a bath if one was constructed with a bit of shovel work. The digging appears good in fine silt & gravel. The map reference for this lower spring which incidentally doesn’t smell as sulphury as the upper one is NZTM2000 E1452034 N5233856. [Topo50 Maps]
Both the springs have small rock cairns nearby (presumably markers).
NB As another correspondent has said it is a serious tramp to reach these springs. Most trampers take two days. Fording of the Mungo River and particularly Brunswick Creek can be problematic even in fine weather.