In the late 1840s or early 1850s, Collett moved to New South Wales, becoming a director of the Australian Mutual Mining Association.[5] In 1854, he was appointed the Commissioner for Roads for the colony, undertaking surveying work on the Northern Road between Morpeth and Murrurundi.[5] In March 1864, Collett moved to New Zealand, becoming the Chief Superintendent of Roads and Bridges.[5] In this role, he facilitated the construction of rail south of Pōkeno, connecting the Mangatāwhiri River south to Meremere, and preparing the initial designs of the Panmure Bridge in Auckland.[5]
Collett was declared insolvent due to financial difficulties in 1866, due to the foreclosure of a gold mining venture at Denison Town, New South Wales which he had financed. He returned to the British Isles,[5] contesting a seat for the Conservative Party for the 1869 Tipperary by-election, however was unsuccessful, receiving only 12 votes.[7]