FAILING to comply with the letter of the law could prove costly for a Gin Gin police officer who may lose the means to continue running his side business eradicating feral pests.

According to his defence lawyer, Senior Constable Matthew Harling, 32, may also face a fine of up to $2500 under disciplinary action from the Queensland Police Service, after pleading guilty to one count of acquiring a weapon.

In the Bundaberg Magistrates Court yesterday, Harling's lawyer said his client was a "police officer of 12 years", and "a long-term weapons holder", who mistakenly believed he had three months to apply for a permit to acquire a double barrel shotgun after taking possession of it.

Already licenced to hold both category A and B weapons, which the shotgun in question falls into, Harling collects and owns 31 registered weapons, largely due to his casual work as a feral pest controller.

Under the Weapons Act, because Snr Const Harling had paid for the gun, instead of just borrowing it on a temporary basis for up to three months, he was required to already hold the permit to acquire when he took possession of the gun.

Harling was tracking the parcel through Australia Post, which was posted to Gin Gin, and on November 6 last year he picked up the package from the post office.

The parcel originally had his name, address and mobile phone number on it, however upon advice from the post office when mailing the shotgun, the seller had instead crossed out Harling's address and readdressed it to the Gin Gin Guns and Ammunition store.

A police investigation was launched when the owner of the store took the parcel slip into the post office and discovered the package had already been collected.

The investigation discovered Harling picked up the parcel in the afternoon and applied online for the permit to acquire the gun at 10.30pm the same day.

Magistrate Belinda Merrin accepted Harling was extremely embarrassed and remorseful about the situation but said because he was a police officer, the holder of a number of firearms and a feral pest controller, it was a "double-edged sword".

"Someone in your circumstance ... ought to have been aware," she said.

"Mistaking the law is no excuse."

The mistake could yet cost Harling his firearms licence and with it his feral pest control business, with his 31 weapons along with the double barrel shotgun seized pending a further investigation following yesterday's outcome in court.

Ms Merrin placed Harling on a $400 good behaviour bond for four months without a conviction being recorded.