A report into the Ministry of Foreign Affairs and Trade's decision to grant disgraced Malaysian diplomat Muhammad Rizalman immunity from prosecution will be released tomorrow.

Rizalman was today sentenced to nine months' home detention after pleading guilty to indecently assaulting Wellington woman Tania Billingsley in May 2014.

Two days after being charged with sexual offences, Rizalman returned to Malaysia, having invoked diplomatic immunity.

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A report into the ministry's decision to grant him immunity from prosecution, completed in December 2014 by former Treasury secretary John Whitehead, has remained secret.

But today a spokesman for Foreign Affairs Minister Murray McCully confirmed the report would finally be made public tomorrow.

"We will be releasing the Whitehead Report within the next 24 hours," said a spokesman for Mr McCully.

Mr McCully would hold a press conference in the morning at Auckland Airport on his way to Waitangi Day celebrations.

Prime Minister John Key has previously criticised the way a ministry staffer handled the affair.

Opposition MPs have criticised the report's delay and were again vocal when approached by NZME this week.

"It should be released as soon as he's sentenced," said Labour foreign affairs spokesman David Shearer.

The report was not about the court case, but the ministry's internal processes.

"It's clearly just Murray McCully trying to hide behind procedure. There must be something in there that's embarrassing for Murray McCully that he's delayed it for so long."

Green MP Jan Logie said the investigation's scope should have included Mr McCully's role in the imbroglio.

"I don't know anyone who understands the rationale as to why they can't release it.

"It was an investigation into the actions of the Ministry of Foreign Affairs and Trade."

Neither Mr Shearer nor Ms Logie had seen the report, but she expected it to point to a number of failings.

Mr McCully's office did not wish to respond to opposition criticism.

After requests from New Zealand, Rizalman returned in October 2014.