Key Points:

Official documents indicate former finance minister Michael Cullen would have been briefed about the billion-dollar blowout at ACC long before the cut-off point for inclusion in the pre-election "opening of the books".

The absence of the blowout from the Pre-Election Economic and Fiscal Update (Prefu) has National accusing Labour of deliberately trying to hide the sum to make the Government accounts look better than they were.

Papers seen by the Herald show Treasury officials were aware of the estimated shortfall in the non-earners account shortly before August 22 and would be "updating the Minister of Finance".

The Prefu was closed off on September 24, with Dr Cullen and Treasury Secretary John Whitehead signing a joint statement of responsibility that all economic and fiscal implications of Government decisions were included.

The blowout took the new National Government by surprise, and Prime Minister John Key has ordered a ministerial inquiry into why the "significant and serious hole" was not included in the Prefu.

Dr Cullen, who signed to having "responsibility for overall integrity of the disclosures", refused to comment yesterday when asked why the ACC shortfall was not included in the Prefu as a "specific fiscal risk".

"I'm the Treaty negotiations spokesman," Dr Cullen said.

"I have no intention of re-engaging in those areas."

Finance Minister Bill English, who is in charge of the ministerial inquiry, said there was an "expectation" that Dr Cullen and others involved would co-operate fully.

The Prefu is required by the Public Finance Act. Offences under the act can carry a penalty of up to a year in prison or a fine of up to $5000 for individuals, or a fine of $15,000 for organisations.

The Prefu's "specific fiscal risks" section contained several potential risks, such as cost overruns in the construction of the Supreme Court.

Risks must be included if they are of "reasonable certainty", going to have an impact of $10 million or more in a year, and if they are under "active consideration" by the Minister of Finance and responsible ministers (such as being subject to a written report).

A Treasury spokeswoman said the ACC shortfall was not included as it was not deemed to be under active consideration.

She would not comment on whether officials informed Dr Cullen as the official papers said was intended. She said the Treasury would not comment further because of the ministerial inquiry.

Former ACC minister Maryan Street, who received her first official advice of the shortfall on August 14, said the Treasury was closely involved and Dr Cullen would have got a "heads-up" from officials.

Ms Street provided the official papers to the Herald, including the August 14 report from ACC that detailed an estimated shortfall of over $900 million over the next three years. The figures were actually estimated out for five years, taking the shortfall to $1.5 billion.

Ms Street said Treasury, ACC and the Department of Labour were all recommending that no decision be made until after the election.

She said that, like her, Dr Cullen was relying on the early estimates, which were not "good figures" and he would have been waiting until she officially wrote to him on October 22 when the figures had been checked out properly.