Thirlmere is a small semi-rural town in the Macarthur Region of New South Wales, Australia. The suburb is governed by the Wollondilly Shire City Council, a local government area of Australia. The postcode of Thirlmere is 2572. It is surrounded by Mowbray Park, Picton, Lakesland and Tahmoor. It is located 89 km away from Sydney.
The Thirlmere area was first discovered by the British in 1798, whose attention was focussed more on the Thirlmere Lakes area and finding an alternate route north towards Bathurst. Thirlmere boomed with the creation of the Great Southern Railway in 1863 to 1867, when the area was blanketed in tents to house the many railway workers that came to the area to work. Thirlmere was valued mostly for the proximity of the Thirlmere Lakes (then called Picton Lakes) which were used to provide water for the steam trains. During this period Thirlmere was also the home for a number of timber mills, whose main product was the milling of sleepers for the railway line.