She was reacting to the release on Thursday of NBN Co's corporate plan for the period 2018- 2021 in which the company outlined its goals for the ongoing rollout, including projections for the first year after what is forecasts to be the completed rollout.

In a statement, Rowland said that Turnbull had promised in 2013 that he would deliver the NBN for $29.5 billion by the end of 2016.

She said in sharp contrast, Thursday's NBN Co corporate plan confirmed that the NBN was now scheduled for completion in July 2020 at a cost of nearly $50 billion.

Commenting on the financial projections by NBN Co, Rowland said at the start of the media conference to announce the plan, "(Communications Minister) Mitch Fifield said 'just because (NBN Chief executive) Bill Morrow and his team looked relaxed, it doesn’t mean that they are' – and they shouldn’t be.

“Malcolm Turnbull’s second-rate NBN is on the ropes. The economics of the multi-technology mix are buckling, and there are some very serious questions NBN Co will have to answer.”

The corporate plan said that the number of premises to be covered had fallen to 11.6 million with fresh geospatial mapping predicting this lower figure. The earlier figure had been 300,000 more.

Rowland said it appeared "that 300,000 homes have vanished from Malcolm's master map. Yet the rollout is not going to be any faster or cheaper. This is remarkable. What is going on here?”

Following up on the release of the corporate plan, the executive director of Internet Australia, a non-profit representing Australian users, called for NBN Co to abandon the use of copper.

Laurie Patton said: "We're pleased to see that NBN Co is clearly moving in this direction."

He said that while it would have been better to have stuck with an all-fibre rollout, fibre-to-the-kerb (what NBN Co calls fibre-to-the-curb) was "a reasonable middle ground. At least it can be upgraded to full-fibre at a later time. When it comes to FttN it's a case of ripping it out at great expense".

Patton also pointed out that in New Zealand, Chorus, which is rolling out that country's broadband network, "has reduced the per premise cost of FttP by around 40-50% as they improve their rollout processes. Surely NBN Co isn't expecting us to believe that they can't do the same?"

He said the way things were going, "whoever is in office in 2020 will have to deal with our biggest ever national infrastructure debacle. NBN Co will owe the government circa  $19 billion and within 5 to 10 years it will need to fund a very expensive FttN replacement".