During the last c. 300,000 years mega-lakes with areas an order of magnitude greater than those of today occurred in the monsoon region of northern Australia. The results of an Atmosphere Global Circulation Model (AGCM) sensitivity experiment show that the repeated occurrence of mega-lake phases during the Pleistocene, and the strong monsoon flow that this implies, can be related to Milankovitch insolation forcing. The AGCM experiment is centred on the Southern Hemisphere low-latitude, summer insolation high of 115,000 years ago. The details of the results indicate a strong response of the Australian monsoon to the higher insolation at that time, with a longer monsoon season, increased precipitation and higher resultant runoff than at present.