The contract allows for two one-year extensions.
Western Power will be one of the first customers to use ASG's new national data centre.
"ASG is recognised as a premier IT service provider in Australia and this new contract demonstrates Western Power's confidence in our expertise, innovation, responsiveness and the high quality of our service offering," said Murray Rosa, ASG Group's chief officer sales and strategic operations.
Western Power CIO Leigh Sprlyan said "Western Power has adopted a multi-sourcing strategy, partnering with leading service providers to provide essential IT services to our business."
"ASG demonstrated the flexibility and creativity we were seeking to support our IT transformation, in the face of strong competition from other key industry vendors," he added.