Telstra and NBN are months away from signing a deal that could be worth between $1.5 billion and $1.8 billion to help build out and speed up the conversion of Australia's cable TV networks.

Both companies on Monday morning announced a series of transactions worth around $280 million over the next four years to help fix faults in the copper phone line network and maintain the $56 billion national broadband network.

Telstra chief executive Andy Penn is set to sign a contract with NBN Co to upgrade its networks by February 2016. Credit:Louise Kennerley

However, the bigger contract win is set to come in February 2016. Both parties have signed a memorandum of understanding and are in the final stages of negotiating a deal that would see Telstra design and project-manage the expansion and upgrade of its hybrid fibre-coaxial (HFC) network.

The HFC networks built by Telstra and Optus are largely used to deliver pay TV and cable internet services. But the Coalition's NBN plans to upgrade and expand it to connect around 4 million homes and businesses with high-speed broadband.