Investigations being carried out by the Department of Mines in the Padthaway irrigation basin and surrounding areas are aimed primarily toward determining the water budget of the basin in conjunction with other Government Departments. The role of the Mines Department in these investigations has been to examine the hydrogeology of the basin with particular emphasis on determining the volume of recharge by groundwater from the Naracoorte Range and the volume of outflow of residual groundwater from the north western end of the basin. With the extension of the Underground Waters Preservation Act over the South East and the introduction of the controls on irrigation in the Padthaway basin, it has become necessary to critically examine applications for permits to drill additional bores and to more accurately refine both the recharge and discharge estimates. Since progress report No. 3 by B.M. Harris was released in June 1972, the Mines Department has continued routine monitoring of water table fluctuations and salinity variations to evaluate the long term effects of irrigation. Further investigations involving drilling, pump testing and discharge measurements have also been carried out. The investigations can be divided into four categories: • The monitoring of seasonal changes in storage by means of routine observations of water table fluctuations • The monitoring of changes in salinity throughout the pumping season • The refinement of discharge estimates • The re-evaluation of recharge estimates involving drilling and pump testing. This report describes the results of these investigations carried out since the issue of Progress Report No. 3. Since the introduction of irrigation controls in the basin, applications for permits to drill new bores have been received, some involving quite large supplies. The current investigations programme therefore assumes increasing importance in defining those parameters that determine the safe yield of the basin.