Delays will not only attend the $36-billion national broadband network (NBN) roll out but also considerable budget overshoots, reports said.

Citing the analysis furnished by telecommunication expert Kevin Morgan, The Australian reported on Thursday that the multi-billion dollar NBN will likely suffer serious implementation glitches as manifested by the adjustments made by the NBN Co in March.

NBN Co, which has been established to manage the federal government broadband project, has admitted in March that its roll out blueprint introduced in December 2010 will miss its coverage targets.

And according to Mr Morgan, NBN Co and Telstra Corporation were running behind the NBN timetable by at least 15 months, no thanks to the project's "fundamental shortcomings and miscalculations."

The delays, Mr Morgan said, will definitely lead to cost blowouts and blames could be pointed to the general policies adhered to by the Labor-led government in making the ambitious project a reality.

Mr Morgan, who The Australian said worked under the Keating government, attributed the budget miscalculations to the absence of previous models that could have guided the framing of the NBN guidelines prior to its actual roll out phase.

"Given the lack of any precedent, the NBN is entering uncharted waters and will find it extremely difficult to deliver all that is being promised and most certainly find it difficult to deliver its network within the promised cost," Mr Morgan was quoted by the publication in a report that he submitted before the parliamentary committee overseeing the NBN roll out.

The roll out plan, as admitted by NBN Co chief executive Mike Quigley, will have to be revised on its targeted premises coverage.

From the original 4.2 million premises by June 2015, the NBN Co said last month that its coverage was pushed back to only 3.5 million premises.

Mr Quigley blamed the downward adjustments to "reasons which we could not control."

Yet in his testimony before the same parliamentary committee earlier this week, the NBN Co boss insisted that pressuring the firm to stay within the original NBN guidance was "neither reasonable nor valid."

He has yet to challenge analysts' projections that the NBN roll out will encounter more snags and will likely extend its coverage to only 2.5 million premises over the next three years, which is a significantly lower target from what the firm has laid out in the original plan.

Mr Quigley's performance so far was assailed by opposition communications spokesman Malcolm Turnbull on Wednesday, stressing that the NBN Co leadership should have updated the corporate plan the soonest they determined it was not workable.

Mr Turnbull strongly reminded the government that the existing NBN roll out plan is the sole basis of how the huge NBN funds will be spent and "it remains the only yardstick against which NBN Co's performance can be held to account until an updated version is released."

Judging from the complications that the NBN Co must deal with, the national broadband project might end up shooting up the cost of internet access in Australia by at least six percent each year, until 2023 when the NBN should have assumed its monopoly of the broadband monopoly, Mr Turnbull warned.

By that time, internet rate access for households would soar by almost a hundred percent, Mr Turnbull said, belying earlier claims by the national government that broadband services would be more widespread and affordable under the NBN.

In a statement, however, Communications Minister Stephen Conroy said that NBN's in-placed retail rates were "demonstrably comparable to, or cheaper than existing broadband prices, for much higher levels of service and reliability."

Also, NBN Co dismissed Mr Turnbull's calculations and told The Australian that "we have frozen the cost of our basic offering for five years and have pledged to limit future increases to below the rate of inflation."