Based on the half-yearly results announced on Monday, the NBN Co is looking at a further cost blowout of $450 million on the HFC network, the Australian Labor Party claims.

Labor shadow communications minister Michelle Rowland (below, right) said in a statement on Monday that the results showed the cost of an HFC installation had risen from $2258 to $2403 per premise.

She said this cost rise, if spread across the 3.1 million premises expected to receive the NBN via HFC, would be expected to deliver a $450 million hit to taxpayers.

Rowland said: "This (the $450 million blowout) comes separately to the $500 million potential revenue hit from the HFC halt announced in late 2017.

In November last year, NBN Co said there were some issues with Telstra's HFC cable system which were bedevilling progress on the rollout.

The company then announced in December that houses and businesses slated to receive the NBN over HFC would have to wait between six and nine months longer for connections.

"Despite this, it remains unclear if consumers and taxpayers are actually getting the complete story," Rowland added. " This is not a multi-technology mix — it’s a multi-technology mess."

Under the revised rollout plan, put in place after the Coalition Government came to power in 2013, the NBN Co planned to use both the Telstra and Optus HFC networks as a means of delivering the NBN, in order speed up the rollout.

The Optus HFC network was later found to be in an unusable state and its use was discontinued. The Telstra HFC network was used until the cessation of connections in December 2017.