A three-layered transient groundwater flow model of the Tertiary Confined Sand Aquifer (TCSA) underlying the South-East of South Australia and south-west region of Victoria has been constructed using MODFLOW software. The model is being used as a predictive tool to provide information about both seasonal and long term changes in the potentiometric head of the aquifer under different extraction scenarios, with the ultimate intention being to determine and risk potential levels of water use from the aquifer. In recognising the importance of treating the TCSA system as a whole, the model domain was designed to include the aquifer's full extent on both the Victorian and South Australian sides of the State Border. The aquifer subcrop area was sub-divided into a number of preliminary management areas, different from those of the unconfined aquifer, but maintaining the Zones of the Designated Area along the South Australia / Victoria State Border. Results from the groundwater usage modelling show that if the presently recommended permissible allocation volumes were adopted, and fully utilised, after 25 years there would be a long term decline in the potentiometric head of the TCSA of between two and six metres. The recommended allocation volumes for groundwater withdrawal from this aquifer are acknowledged to be conservative, but the conforming groundwater model results clearly reflect the overall sensitivity of the confined aquifer system to such extraction.