The company said in a statement it had worked with Ericsson and Samsung, using mid-band spectrum and 5G carrier aggregation to achieve this speed at a live site in Sydney.
A total of 150 such ultra-fast sites are planned in Sydney and Melbourne by the end of November on the telco's existing 5G infrastructure.
"As they roll out, these new ultra-high-speed sites will provide a real speed boost to Optus customers with capable 5G handsets and 5G SIM," Optus managing director, Networks, Lambo Kanagaratnam, said.
He described speed as a critical component of 5G, but added that "not all speeds are created equal".
"Optus 5G ultra-high-speed sites are the next step in our ambition to deliver Australians a network that offers the fastest 5G. This is not lab technology, this technology is available now and we have an ambitious rollout plan over the next few months to ensure that more of our Optus 5G Mobile customers benefit from these 'gigabit' speeds."
Kanagaratnam said the company was also looking to provide its home Internet customers with high speeds and planned to unlock 100Mhz of 3500Mhz mid-band spectrum for users in selected areas of Sydney and Melbourne.
The Optus 5G Home Internet service is already delivering average download speeds of 214Mbps and it is claimed this increase in spectrum bandwidth of around 60% will mean even faster average speeds.
"We see our 5G Home Internet as Australia's best and it's already fast – in fact, in recent weeks we have seen more and more customers experience download speeds in excess of 300Mbps and with access to this 100Mhz mid-band spectrum we expect to see these speeds climb even higher," Kanagaratnam said.
"We lead the market for 5G Home Internet and having been available to customers for more than a year, we understand the way our customers are using this product and with multiple devices all streaming content simultaneously, we know that this speed boost will be welcomed."
Optus has 920 5G sites and its network can be accessed by 650,000 Australian homes.