NBN Co has blamed work "inherited" from Telstra after embarrassing footage emerged of cabling hooked up around two trees in Sydney's south-west.

Key points: The video clip posted on Facebook is approaching half a million views

The video clip posted on Facebook is approaching half a million views The NBN has inherited the Telstra fixed-line network as part of the NBN rollout

The NBN has inherited the Telstra fixed-line network as part of the NBN rollout Telstra conceded it should have gone back and completed the job

A 40-second clip posted to the "Humans of Bankstown" Facebook page on July 7 shows cabling that was supposed to be underground strung up and across two trees on a suburban street as a means of circumnavigating access issues.

The person filming the clip joked it was the "smart" solution for when pits — the pavement-level access points to the underground telecommunications network where cables can be joined and connected to households — were blocked.

The video has been viewed about 480,000 times.

An NBN spokeswoman said the issue had been discovered while rolling out the NBN in Hurstville.

It could not confirm when the clip was filmed.

It said the technician featured in the video was assessing the cable, not installing it.

"The cabling was not installed by NBN," the spokeswoman said.

"We inherited the network from Telstra in that condition and believe the cabling between the two trees had been like that since or before July 2015.

"As there was no impact to service quality of the cabling, the non-compliant network set-up was not picked up in the design process or in network testing."

The NBN's agreement with Telstra on the separation of the telco's retail and network businesses means the fixed-line network has been taken on by the NBN in any existing condition, with repairs undertaken as and when needed.

The spokeswoman said problems picked up during the rollout like this were "very rare".

"We are working to rectify the issue," she said.

"It is a unique and complex situation so we cannot provide a timeframe for the works to be completed."

A Telstra spokesman said the cable in the trees appeared to be a temporary fix that had not been followed up and completed properly.

"Our focus at the time was on keeping our customers connected, but unfortunately we should have gone back and completed the repairs," he said.

He added it was difficult to determine how long the temporary fix had been in place, but it would be updated with the NBN rollout in the area.

The national NBN rollout is expected to be completed next year.