Investment in telecommunications networks could soon become unsustainable without major reform to the sector, such as changes to NBN pricing, fairer spectrum allocation and tax incentives, Optus chief executive Kelly Bayer Rosmarin has warned.

Speaking on Monday, Ms Bayer-Rosmarin said Optus was in the middle of a major capital investment phase as it built out its 5G network and prepared to bid for a large chunk of the millimetre wave spectrum necessary to transmit 5G.