As part of the deal Optus will migrate the department's Wide Area Network (WAN) to the Optus Evolve network to enable unified communications. DIAC mobile staff will use the Optus 3G network and wireless broadband.
Network monitoring and capacity planning will be supplied as part of the deal.
DIAC CIO Bob Correll said in a statement the contract will future-proof the department's networks and telecommunications.
"The solution Optus proposed and is currently implementing will offer us the flexibility and scalability we require to meet changing business demands, both now and in the future," Correll said.
Optus has been a DIAC supplier since 2001.