The Coalition Opposition has siezed upon the shortfall saying it is evidence that Government is recklessly flying by the seat of its pants economically.
'Considering the planned $43 billion expenditure on the NBN - and perhaps more - we call on the Treasurer to urgently submit the proposal to Treasury for a full costing,' a spokesman for Mr Robb told iTWire.
'We think this problem has been highlighted by the fact there is no real business plan for the proposal, no cost benefit analysis.
'It really illustrates that the whole thing has been cobbled together quite recklessly.'
The Government also targeted Prime Minister Gillard, claiming she is ignorant of the details concerning the structure of the NBN and its costings.
'Even today Julia Gillard was asked about specifics of the network in relation to how many households will be connected in the next term of the Gillard Government should they be elected and she had no idea,' the spokesman told iTWire.
'She was asked about what customers would have to pay to access the service and she had no idea.
'Considering the expenditure and now the new doubt that has been raised about the Government's financing (of the NBN) it should be submitted for a full and thorough costing by Treasury which to this point in time the Government has refused to do.'
Responding to claims of the funding shortfall, representatives of the Labor Government today questioned the validity of the Parliamentary Library report and criticised the accuracy of its findings.
However, Mr Robb's office dismissed the criticisms and repeated the call for full Treasury analysis of the Government's NBN costings.
'We would expect the Parliamentary Library to do extremely thorough and accurate work,' Mr Robb's spokesman told iTWire.
'In this particular instance, they've done an admirable job in trying to piece together all the various financial pieces of the puzzle.
'The thing is that the Government has put so little information out there as to how the Government is going to pay for the thing, the Parliamentary Library has faced quite a difficult job in doing that.'