Suggesting that the Standards In Public Office Commission decision was not unanimous, Green Party leader Trevor Sargent called for reform of the laws governing ethics in politics and said it was not good enough that a consultant should be able to do fundraising for a minister.
Describing Mr Cullen as the "prodigal minister", Mr Sargent queried how the Minister for Transport could claim to be exonerated when there was so much criticism of the appointment.
"I am not surprised the SIPO found no prima facie case insofar as Dermot Quigley's report was quite rushed. I wouldn't say they had an agreed position but they put their decision into the public domain," he said.
But Taoiseach Bertie Ahern said the SIPO found no basis for investigation.
"There is nothing to suggest Minister Cullen acted inappropriately," he said.
Referring to the recommendations in the report by former Revenue Commissioner Dermot Quigley, Mr Ahern said the Government planned to announce the reforms of the procurement system very shortly.
"The Government discussed the matter and we agreed to include it into the Cabinet handbook," Mr Ahern said.
Also yesterday, Green Party environment spokesman Ciaran Cuffe questioned the Government, seeking details on any planned changes.
"I think this investigation has shown there is a clear need to reform the legislation on ethics in public office, so that the SIPO is given a bit of clout and where there is the perception of impropriety, or impropriety, that there should be a clear investigation," he said.