Colless won a cadetship with the Soil Conservation Service of the New South Wales, that led to studies at Hawkesbury Agricultural College.
He became Chairman of the New England Electorate Council in 1999 and served a term during 2000 on the Central Executive of the National Party.
He was appointed to the Legislative Council on 30 August 2000 following the resignation of the National Party MLC Richard Bull.
Colless served as Chief Whip for The Nationals in the Upper House from 2002 until 2015, chaired the Standing Committee on State Development from 2011 to 2015 and was appointed as the Parliamentary Secretary for Natural Resources and Regional Planning from March 2015.
[2] John Hargrave was also appointed as a Representative of Government in the Legislative Council and served as Solicitor-General and Attorney-General until 1865.