The Office of the Auditor-General says a major overhaul of Inland Revenue's new information technology system has to be a success.
Cabinet has so far approved $193 million for the programme but that's expected to grow to $2 billion.
The Auditor-General's David Press told Parliament's Finance and Expenditure committee risks had to be managed, to ensure the new system was economic and efficient.
"Ultimately this is a programme that cannot fail because there is so much dependant on it and a key building block of effective programmes is good governance."
Mr Press said the Auditor-General would continue to monitor Inland Revenue's progress.
Inland Revenue said it was very aware of managing the programme costs and the department had a good track record.
Several MPs questioned Inland Revenue about value for money, citing the cost blowout on its revamp of the child support payment system.