Colless was born in Uralla, a small town in the Northern Tablelands of New South Wales.

During that time he conducted mosquito research with a special focus on actual and potential disease vectors.

In 1960 he returned to the CSIRO where worked as taxonomist at the Australian National Insect Collection (ANIC) until he retired in 1987.

Colless described the two fly families Perissommatidae and Axiniidae, 13 new genera, including Valeseguya, Perissomma, Colonomyia, and Chetoneura as well as 120 new species, including the mosquitoes Aedes malayensis, Anopheles saungi, Anopheles stookesi, Culex alienus, and Culex pseudovishnui.

[1] His most cited[2] and perhaps most utilized contribution is entitled Congruence between morphometric and allozyme data for Menidia species: a reappraisal,[3] in which he devised the calculation of the consensus fork index (CFI), which is sometimes referred to as Colless' index.