"Past delays cannot justify panic or cut corners now. Mistakes we make in the design and/or policy settings for the proposed NBN, particularly in the areas of structure, affordability and accessibility, will not be easily fixed down the track and could be disastrous for our international competitiveness."

According to the AAB, allowing the Productivity Commission to assess the Federal Government's NBN proposal would inject facts into a broadband debate where they were otherwise missing.

"The NBN proposal now differs significantly from the proposal that was contemplated in the Implementation Study (which already found that the proposal in its then form was unlikely ever to generate a commercial rate of return)," the letter reads. "No serious analysis has been done of the changed cost implications of those differences. Fundamental features of the project remain unresolved."

The letter also echoed Opposition communications minister, Malcolm Turnbull's that the NBN Co's plan to limit the number of points of interconnect (PoI) on its network will "strand" hundreds of millions of dollars of existing private investment in fibre links.

"NBN Co's suggestion that it would prefer a small number of Points of Interconnect nationally has been met with almost universal opposition from the industry, will strand billions of dollars in private investment and thousands of kilometres of optical fibre, and lead to network inefficiencies."