Ozaviation is 95% correct, they left out some aquisitions.
In 1983 Skywest bought 100% of HC Sleigh Aviation and Sleigh Airlines. The Sleigh Group started life as Executive Airlines in Essendon who originally held the VIC/TAS aerial Amubulance contracts with all the VH-EX... Aerocommanders.
HC Sleigh Aviation had the Customs contract Australia wide with Nomads and Coastal Surveillance Contracts from Millingimbi and to the East. Skywest already had Darwin and WA Coastal Surveillance.
Skywest immediately sold Sleigh Airlines who operated from Essendon to Tasmania including 3 hangers at Essendon.
Out of this sale, Airlines of Tasmania was formed, Esso started their own service from Essendon to Sale with a Citation instead of the Baby Metro and Steve Nott with backing from the original owner of Executive Airlines started GAM at Essendon.