Delays with works conducted by one of its major subcontractors has seen the NBN Co downgrade its June 13 target for the number of premises passed for connection to the service.

NBN Co CEO, Mike Quigley, told a Senate Estimates hearing last week that the company was reducing its target for June from 300,000 to 286,000 due to ongoing delays with construction subcontractor Syntheo.

Syntheo is currently working to rollout high-speed fibre optic cable to premises Western Australia, South Australia and the Northern territory.

Mr Quigley told the hearing that the NBN Co was working to recoup its original forecasts with the rollout with Syntheo, but admitted that the company had not yet connected any fibre serving area modules in WA or NT.

Syntheo, a joint venture of Lend Lease and Service Stream, can face penalties for failing to meet rollout targets set by the NBN Co.

The company currently has $315 million worth of contracts with NBN Co, which are due for renewal this year, with an option of extending upgrades to the tune of an extra $510 million.