The independent police watchdog is investigating the arrest of a man whose farm was seized under the Proceeds of Crime Act, after claims of corrupt police conduct, reports Emily Watt in today's Dominion Post.

Shaun Allen had owned his 840-acre farm for just a few months when police raided the property and found 1,000 cannabis plants.

Mr Allen, who served an 18-month sentence and had his $365,000 Hawke's Bay farm seized after he was convicted of growing marijuana in 1993, has spent 15 years and a $500,000 Lotto windfall after he came out of jail fighting to clear his name.

He says he found evidence of doctored documents and manufactured evidence in the police case.

He is planning to apply for a prerogative of mercy, in which the governor-general pardons a person who has been wrongly convicted.

Mr Allen has always maintained his innocence and says the crops were being grown by a previous owner or a trespasser. He thinks he was set up by police determined to convict someone. With the help of a team of lawyers and private investigators, he says he has uncovered evidence of corrupt police practice.

The Independent Police Conduct Authority has confirmed it is investigating. Police did not return requests for comment.

National MP Chester Borrows, a former police officer, said he had seen Mr Allen's evidence and described the case as "very concerning".

"There are documents here and procedures here where police have worked well away from the norm," he said.

Mr Allen said he had spent "many hundreds of thousands" of dollars in fighting to clear his name.

"But it's not the money - it's the emotional part that's the worst part. It is horrible what it's done to me. I used to be a strong person. It's knocked me around."

Mr Allen's farm was the first property seized under the Proceeds of Crime Act in 1993. Since then, the Government has seized lifestyle blocks, Rolex watches, gold bullion, tropical fish and cars.

The act allows courts to seize assets bought with the proceeds of crime, sell them and the money given to the Crown.

He admits he sold cannabis in his youth, but that was in the past.