Telstra is investigating whether it should copy TPG Telecom's plan to compete against the national broadband network in a move that could break NBN Co's business case and add billions of dollars in expenses to the Federal Budget.

The Australian Competition and Consumer Commission (ACCC) on Thursday allowed TPG Telecom to continue connecting urban apartments to its network, which competes against the national broadband network.

NBN Co. has argued that TPG’s plan to connect homes and businesses with fibre to the basement internet services could hurt its business case. Credit:Glenn Hunt

TPG launched a plan in September 2013 to connect 500,000 urban apartments and businesses to a fibre-to-the-basement (FTTB) network that provided internet speeds similar to those offered by the NBN.

The company used a clause that allowed networks built before 2011 to be extended by 1 kilometre to go around laws designed to stop NBN Co from having any competition.