In a statement, Greg Spears, the company's executive manager for Corporate Media, said there were 139,263 premises that were on FttN and could not reach a download speed of 25Mbps.
He said this was about 4% of the total FttN services that were connected, as approximately 3.05 million premises were connected to the network through this technology as of 30 June.
On Monday, Labor shadow communications minister Michelle Rowland cited the promise made by the Coalition Government in 2013, when it said at its election policy launch: "Our goal is for every household and business to have access to broadband with a download data rate of between 25 and 100 megabits per second by late 2016."
Spears said: "NBN Co has made positive progress in the last four months, improving the speed of broadband services on FttN, in particular, so that an additional 14,000 of the premises identified in May 2020 can now receive at least 25 Mbps.
"We recognise that we have more work to do to deliver download speeds of at least 25 Mbps to the remaining 126,079, or approximately 4% of premises, on FttN that are receiving less than 25Mbps and we are undertaking this work as a priority.
"One of the main reasons that some homes and businesses connected to the NBN are not receiving the minimum peak 25Mbps download rate [is because] where co-existence is still in place.
"Once all legacy services are removed from the area, co-existence can end, enabling the node to be reconfigured. Other common issues include line faults and wiring issues in customers’ premises, which we are working with retailers and customers to address.
"Currently approximately 30% of FttN services have reached the end of co-existence with legacy telecommunication services, enabling NBN Co to reconfigure the node and utilise the full speed capability of the line.
"We forecast that the end of co-existence will be fully realised in FY24, and we are working to bring forward the end of co-existence where possible."