Police say they're satisfied the firearm was not purchased for any sinister purpose and that's why the investigation's only resulted in the issuing of warnings.
Auckland City district commander Richard Chambers dismissed claims that pointing out the loop-hole in the law was of public interest and therefore MediaWorks staff should not have been investigated.
"Police would like to make it clear that for any investigation, public interest considerations are applied at the conclusion of an investigation and in accordance with the Solicitor-General Prosecution Guidelines, when prosecution is being considered," he said.
Police view this case as no different to any other matter where criminal offending is disclosed. The circumstances of individual cases are routinely assessed to ensure that an appropriate investigation is initiated.
We would also like to be clear that the freedom of journalists to report on any matter is fully accepted without question by Police.
The law, however, applies equally to everyone, including members of the media and Police do not accept that it is appropriate to commit a criminal offence purely to publicise the ease with which something can be done."
Du Plessis-Allan spoke about the matter on her show Story this evening.
"We want to say thank you to the police. We understand why the police needed to investigate and we welcomed the outcome of the investigation but most of all we want to thank the police for so quickly shutting down the flaw we exposed.
"They did this within hours of us calling them in October and that was the point of our story, so thank you," she said.
Duncan Garner, her co-host, added that the police decision was a "victory for common sense".
"It was in the public interest."
In October, Du Plessis broke a story highlighting that a rifle could be purchased through mail order, apparently without producing a gun licence.
The mail order form sent to Gun City included false details, such as an invented gun licence number.
A police investigation was launched immediately into allegations "false details had been used to fraudulently obtain a firearm via an online/mail order dealer".
In a statement, police said possession of a firearm without a licence could lead to three months in prison and a fine of up to $1000. However, the investigation also looked further into charges of "obtaining by deception" which carried a maximum of seven years in prison.
Du Plessis-Allan said she had sleepless nights after police searched her Wellington home during their investigation.
"What does it feel like to have your house searched by police? Well, we did know yesterday that it was going to happen, but that doesn't give you the time to prepare. It just gives you a sleepless night.
"You might have nothing to hide, but you have so much - trust me - that you want to keep private. The police went through all my stuff. They know what's in my bedside drawer, they've looked inside the boxes under my bed, they've seen the receipts showing what I bought my family for Christmas."
Du Plessis-Allan's husband, Newstalk ZB journalist Barry Soper, tweeted a photo of police searching their home for handwriting samples.
