In 1831 Major Thomas (later Lt. Col. Sir Thomas) Mitchell, Surveyor-General of New South Wales, discovered the lower courses of the Peel (Namoi), Gwydir, and Dumaresq Rivers and identified the Upper Darling. In 1833 he explored the area between the Bogan and Macquarie Rivers and in 1835 traced the Darling 300 miles down from Bourke.

In 1836, following the course of the Lachlan, he reached the Murrumbidgee, the Murray, and the junction of the Murray with the Darling. Returning up the Murray, he turned south and discovered the Avoca and Glenelg Rivers. He then went on to discover the Campaspe and Wimmera Rivers and reached the coast near Cape Northumberland.

In 1845, with Edmund Kennedy as his second in command, he discovered the Warrego, Belyando, and Barcoo Rivers. His mission was to find a river running into the Gulf of Carpentaria, and, although he was unsuccessful, his exploration led to the opening up of the rich pastoral areas of Central Queensland.