Lonsdale arrived in Sydney on 14 December 1831 with a detachment of troops guarding convicts on the "Bussorah Merchant". During the next five years he served in the colony of New South Wales, his brother serving in Van Diemen's Land before joining him in New South Wales in 1833. Whilst stationed at Port Macquarie, William was able to purchase his commission as a captain, and married Martha "née"Smythe on 4 April 1835. He returned to Sydney Town and on 2 January 1836 was gazetted as a Magistrate of the Territory and also as Assistant Police Magistrate. William and Martha's first child, Alice Mary, was born at Parramatta on 20 February 1836.