Police have ramped up efforts to seize ill-gotten gains from criminals - this week adding another $6m of property from a raid on the Head Hunters to $600 million worth of assets and cash restrained since 2009.

Police restrained $167.5 million worth of assets nationwide in the 12 months to June 30 this year, $50 million more than any other year since the Criminal Proceeds (Recovery) Act came into force in 2009, documents released to the Herald show.

On Monday morning, police swarmed the Head Hunters Motorcycle Club in Ellerslie and restrained $6 million worth of property under the Act.

Known as Operation Coin, police were granted orders from the High Court to restrain the properties controlled by a senior Head Hunters member, a 62-year-old beneficiary.

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The Act essentially forces someone to prove legitimate acquisition or purchase of the goods in question, and where the money came from.

Restrained assets not meeting the threshold of legitimate purchase could be forfeited - legal ownership transferred to the Crown - on the civil standard of proof, "the balance of probabilities". This was a lower level than the criminal standard of "beyond reasonable doubt".

Those assets were then sold, and the proceeds were distributed to police and other agencies.

Restraints made in Auckland alone last year totalled $137m, more than 80 per cent of the total nationwide.

Currently, $342m worth of assets were in control of the Official Assignee, the office in charge. These included assets such as artwork, jewellery, boats and farms, as well as property, cars and cash.

Last year, 381 assets worth $61.6m were officially forfeited to the Crown.

President of the Criminal Bar Association Len Andersen said the act had been an effective tool in seizing assets without the difficulty of securing a criminal conviction.

"It has certainly been successful in cases where people have embarked in criminal activity, amassed wealth, and it has not been possible to prove the link between the two," Andersen said.

"Is it right or wrong? Well, it's a value judgment for people. I can't imagine anybody would get too upset at stuff being seized from people like the Head Hunters. I'm not aware of this act being used in a situation where it might have been perceived to cause injustice. But certainly the theoretical potential is there."

Detective Superintendent Iain Chapman, who was at the Head Hunters headquarters on Monday, said the matter was a civil investigation and at this stage no charges or arrests had been made.

"Today is the combination of a very long investigation involving the asset recovery units of NZ Police.

"We have been looking at the Head Hunters gang and in particular on a member of the gang and as of this morning we have taken action under the civil recovery act to be able to look at restraining assets belonging to that individual," Chapman said.

"This is a restraint process and there is no criminal process attached to this.

"This is entirely in the civil jurisdiction and we will now go through a proceeding where we aim to forfeit these assets."