Optus is believed to be paying a rental of about $4,000 per square metre to occupy space on the ground floor of the Royal Mail House building at 253 Bourke Street.
Telstra vacated 253 Bourke Street earlier this year, after quietly doing a deal with Nike, across the road, to take over part of its prominent retail outlet at the corner of Swanston and Bourke streets.
Nike occupied that property for eight years and had six years left to run on a lease, when it struck the deal with Telstra in 2007.
Allard & Shelton directors Patrick Barnes and Joseph Walton negotiated the 253 Bourke Street lease. It confirmed Optus is the retail tenant, but declined to comment on terms.