The New Zealand government has admitted on Tuesday that Megaupload founder Kim Dotcom was a subject of surveillance operations conducted by NZ's domestic spy agency in 2011.

NZ Prime Minister John Key also disclosed that he was informed only of the intelligence operations - considered illegal under New Zealand's laws - Monday this week. A month had past after his deputy, Foreign Minister Bill English, was formally notified on the matter.

At that time too, Mr English signed a suppression order that would have kept the public from knowing that Mr Dotcom was spied upon by the NZ Government Communications Security Bureau (GCSB), a government agency supposedly tasked to snoop on foreign nationals.

The operations, which took place from July through December 2011, were carried out without the support of a government ministerial warrant, The Herald Sun reported on Tuesday.

The News Ltd publication also said that even with a warrant backing the bugging operation, it should have been the task of the NZ Security Intelligence Service (SIS) since Mr Dotcom and another of his colleague, Bram van der Kolk, are New Zealand residents.

Local media reports have also indicated that the GCSB was apparently clueless of its illegal activities against the Megaupload owner and only sought ministerial certificate of its actions when the defence team of Mt Dotcom started raising questions.

According to Fairfax New Zealand, Paul Davison, Mr Dotcom's lawyer, had demanded in August for the identity of the men observed on client's residence prior to the raid conducted by NZ police authorities and agents from the U.S. Federal Bureau of Investigation (FBI).

In the immediate aftermath of the court hearing, Mr English signed a ministerial certificate and was doing so in the absence of Mr Key, who was then in United States for a holiday, Fairfax said.

On Tuesday, Mr Key told Fairfax that the document signed by his deputy was "in relation to information about whether the bureau has acted, because a court, or someone might ask, for that information."

What was executed was essentially a suppression order, Mr Key said, adding that it was not unusual that he was not informed by Mr English on the matter.

Nonetheless, an inquiry has been ordered by his office, the prime minister said.

In an interview with The New Zealand Herald, Mr English said his actions last month were part of "an administrative procedure related to the court."

He added that since an inquiry has been ordered by Mr Key, all the questions on the matter will be duly covered and dealt with "and it's a matter before the courts."

But the Labor opposition viewed the revelation as "a shocking breach of New Zealand's very strict laws restricting the ability of our spy agencies to snoop on people."

"This is a government which is obviously not talking to itself, not talking to the people who are serving it, not taking the responsibility for ministries in which they are responsible for," Labor leader David Shearer told Fairfax on Tuesday.

What just unfolded is "a complete debacle," Mr Shearer added.

Mr Dotcom reacted to the news by posting his sentiments on Twitter, Stuff.co.nz reported on Tuesday.

"We came to NZ to raise our young family in peace. The Government greeted us with unlawful acts, abuse and terror while Mega was totally legal," the German national was quoted by the tech news site as saying.