Starting mid-September, the two cities join Darwin, Perth, Fremantle, Mt Isa, Mildura and Griffith, where the 700MHz spectrum network was switched on in July.
The freed up 700MHz spectrum is scheduled to be switched on across the country in January 2015.
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Telstra and Optus were granted early access commercial licences by the Australian Communications and Media Authority (ACMA) in July for the low band 700MHz spectrum, which allows signals to travel further and reach deeper into buildings.
Mike Wright, Telstra's Group Managing Director Networks, said the expansion of commercial trials in Sydney and Adelaide is an important step in the development of the new network.
"We are excited to offer customers in these parts of Sydney and Adelaide, with compatible devices, an early view on the future of 4G technology and the opportunity to experience the fastest mobile data speeds in Australia," said Wright.
"Telstra also has double the holdings of 700 MHz spectrum than the other carriers, and, this combined with our existing 4G services, doubles the 4G capacity of our network in these areas, and means we can support more customers, using more devices, doing more things at the same time."
It is expected that more compatible devices will make its way to Australia before the end of the year.