In a statement, the company said it would deliver all fixed and mobile voice, mobile data including bulk SMS, unified communications and collaboration, contact centre and network management services on behalf of the ATO.
Optus Business managing director Chris Mitchell said: "The nature of service bundles awarded to Optus highlights that ATO is looking to Optus to build value on top of the network.
"These higher value-adding services enable the ATO to speak to their customers, employees and other stakeholders, and to enable secure communication in and between applications and devices.
The company pointed out that the ATO's contact centre operation were among the most advanced, with call volumes as high as 100,000 a day.
Optus Business said these operations were enabled through advanced call routing, an award-winning digital agent ("Alex") and the biggest pool of voice biometric data to deliver enhanced outcomes for ATO customers and reduce fraudulent activity.
"As a key partner and contributor to ATO's technological transformation journey over the last 10 years, Optus has celebrated ATO's many successes and advances. These new contracts will allow Optus to continue supporting ATO in its ongoing innovation," said Mitchell.
Highlighting the tax office's increased emphasis on cyber security, he added: "Information privacy, system security and scam prevention were central to our many discussions during the process.
"The ATO and Optus Business will continue to focus on security as a core component to the ongoing design and delivery of services to ensure the continuous protection of the ATO, its data, and its interactions with Australian citizens and businesses.
"The ATO's decision is yet a testament of Optus' transition from a traditional telecommunications carrier to a modern technology provider with the ability to build and deliver value on top of the network."