National Broadband Network's (NBN's) purchase of the Optus cable TV and broadband network for A$800 million (around $578 million) in 2012 may not have been a wise decision after all.

Per a leaked document, Australia's NBN is considering rebuilding the Optus network completely as it is "not fit for purpose" and will be investing another A$375 million ($270.8 million) on the project. NBN needs to rebuild the Optus network as it is not able to deliver high-speed broadband, a service NBN has been promoting among Australia's residents and businesses.

The information has been divulged in a leaked internal NBN presentation draft, which reveals that Optus network is incapable of supporting NBN's services. Therefore, it must either be replaced or upgraded and this would cost nearly $375 million.

The leaked document is marked "confidential" and is dated Nov. 3, 2015. The document also suggests moving to "Plan B" for the hybrid fiber-coaxial (HFC) network instead of upgrading the Optus network as originally intended.

The leaked document, which was meant for senior NBN executives, also shares that some of the equipment are nearing the end of their life cycle and need replacing, whereas some are oversubscribed. Optus' network is required to be "overbuilt" to encompass 470,000 premises per the document.

The document also notes that the intended works would cost a significant amount of time and money. The cost of replacement or upgradation of the network would cost $375 million.

"NBN will miss its financial year 2017-18 ready-for-service targets by respectively 300,000 and 333,000 [premises]. Preliminary analysis indicates a peak funding increase of $150 million to $375 million," states the leaked document.

This suggests that the 470,000 premises would not get the service prior to 2019.

NBN has denied the claims that the project requires additional funding and the company is optimistic of meeting its long-term goals.

"Our corporate plan has accounted for the ebbs and flows expected in a project of this scale. NBN has met or exceeded all targets over the past 18 months and we remain confident in our long-range plan and the various strategies we have in place to manage the risk," notes a statement from the company.