From their web site: They look pretty confident about the water,...

From their web site: They look pretty confident about the water, and as redux post shows SO4 in the same area have plenty it seems:
Goldphyre considers the Lake Wells Potash Project to be unique in that it is the only Australian brine potash project with extensive historic¹ and recent drill coverage over the central portion of the target area. Detailed logs from gold and base metal exploration work dating back to the 1990’s have recorded water inflow, water table data and lithological information which has been used to generate a first pass aquifer volume model (Table 1, Figure 3) to as a basis for a resource target.
Four wide spaced lines of AC drilling were used for the model with the upper surface fixed to top of water table or first damp/wet sample in hole and the lower surface fixed to Archaean/hard rock basement. This drilling data has revealed an interpreted deep paleochannel in the central part of the project area (+140m deep). This deep palaeochannel may not only play a critical role in recharge of the near surface brine observed in pit sampling but also add substantial potash brine volume potential.
This historic Western Mining Corporation’s (WMC) drill data enabled Goldphyre to model the aquifer and calculate a volumetric estimate for the aquifer at over 1.6 billion m³."