results of the expedition were to reveal the pastoral wealth of the Murchison and Champion Bay districts and the discovery of a lode of galena in the bed of the Murchison river. Later in the same year Gregory accompanied the Governor, Capt. Charles Fitzgerald, R.N., on a visit to the mineral discovery, which proved to be of more importance than was at first supposed.
In 1855–6 Gregory undertook an expedition under the auspices of the Royal Geographical Society with the dual purpose of exploring the previously unknown interior of the northern territory of Australia and searching for traces of the lost explorer Friedrich Wilhelm Ludwig Leichhardt [q. v.]. Starting from the mouth of the Victoria river, the party ascended that river to its source, crossed the watershed to the southward-flowing Sturt creek, and then made its way to the gulf of Carpentaria and thence to the Dawson and across the northern peninsula to the east coast. The result was the shedding of much light on the rivers of this region, the discovery of the water parting formed by the Newcastle ranges, and the charting in sixteen months of 5000 miles of hitherto unknown wilds, but no certain traces of Leichhardt were found. For his achievements on this expedition Gregory was in 1857 awarded the founder's medal of the Royal Geographical Society.
In 1858 he undertook his last exploring expedition, when he was despatched by the New South Wales government to renew the search for Leichhardt. He started from Sydney on 12 Jan. and reached the Barcoo in April. In latitude 24° 25′ and longitude 145° S. he found a tree marked L and some stumps of others which had been felled with an axe. In May he reached the Thompson river, and followed it till it ran out in plains of baked clay. He then pushed down Cooper and Strzlecki Creek, and arrived at Adelaide after a seven months' exploration, which left the fate of Leichhardt as much in doubt as ever.
On his return from his last expedition he was employed in defining the southern boundary of Queensland, and became surveyor-general for the new colony, a post which he held from 23 Dec. 1859 to 11 March 1875. Thenceforward until 1 Sept. 1879 he was geological surveyor of the southern district of the colony. On 10 Nov. 1882 he was nominated a member of the legislative council, but did not take his seat till 26 June 1883. He played a prominent part in the debates, his intimate knowledge of the country and its resources and his fund of scientific and other information securing him an attentive hearing even from those who differed from him. It was his custom to sit always on tho opposition benches, in order that ho might be more free to criticise the various government measures.
Gregory took an active interest in municipal affairs. He was one of the first members of the Toowong shire council, and when the shire was gazetted a town in 1902 he was chosen first mayor. He was a trustee of the Queensland Museum from 1876 to 1899, and from 1876 to 1883 sat on the commission to inquire into the condition of the aborigines.
He took a keen interest in scientific work of all kinds, and in 1895 was president at Brisbane of the Australian Association for the Advancement of Science, devoting his opening address to a sketch of the geological and geographical history of Australia.
He was created C.M.G. on 27 Feb. 1875, and K.C.M.G. on 9 Nov. 1903. He died unmarried on 25 June 1905 at his residence, Rain worth, Brisbane, and was buried in Toowong cemetery.
Gregory, according to Sir Hugh Nelson, 'contributed more to the exact physical, geological, and geographical knowledge of Australia than any other man, for his explorations have extended to west, north, east, south, and central Australia.' He was joint author of 'Journals of Australian Exploration' (Brisbane, 1884) with his brother, Francis Thomas Gregory (1821–1888), who was in the survey office of Western Australia from 1842 to 1860; Francis accompanied his brother Augustus in his first exploring expedition in 1846, and led two expeditions himself in 1858 and 1861, being awarded the gold medal of the Royal Geographical Society in 1863 ; going to Queensland in 1862, he was nominated to the legislative council in 1874, and was for a short time postmaster-general in the first McIlwraith Ministry.
[The Times, and Brisbane Courier, 26 June 1905; West Australian, 27 June 1905; Geographical Journal, vol. 26, 1905; Western Australian Year Book for 1902–4; Mennell's Dict. of Australas. Biog., 1892; Burke's Colonial Gentry, 1891; Favence's History of Australian Exploration, 1888; Blain's Cyclopædia of Australasia, 1881; Heaton's Australian Dictionary of Dates, 1879; Howitt's History of Discovery in Australia, vol. ii. 1865; Tenison Woods's History of the Discovery and Exploration of Australia, vol. ii. 1865.]