The Optus cable TV and broadband network bought by the National Broadband Network for $800 million is in such poor condition the NBN is considering replacing it entirely.

Replacing the network would see costs blow out by up to $375 million and 600,000 premises forced to wait until 2019 before connecting to the NBN, according to documents obtained by Fairfax Media.

The NBN's Hybrid Fibre Coaxial (HFC) rollout will use existing pay TV cables to deliver high-speed broadband to Australians.

The leaked briefing presentation, dated November 3, 2015 and marked "confidential", canvasses moving to a "Plan B" for the HFC network, rather than upgrading the Optus network as had originally been planned.