Nearly $20 million flowed into bank accounts linked to Head Hunters boss Wayne Doyle over the past 15 years, according to the police.

Half of the money was banked in cash and $9.2 million was deposited into the account of the motorcycle gang's charitable trust, which suggested the entities linked to Doyle were money laundering, said the Chief High Court judge.

Over that time, investigators from the Ministry of Social Development allege Doyle claimed $380,000 in social welfare payments he was not entitled to.

The forensic analysis formed part of the police case, which satisfied Justice Geoffrey Venning there were grounds to grant orders freezing $6m worth of assets linked to the gang president.

Doyle, 62, is widely considered to be the godfather of the feared gang, which has aggressively expanded across the country in recent years.

Other senior Head Hunters have been jailed in recent years for serious methamphetamine charges, but Doyle - who was jailed for murder in the 1980s - has not been convicted of a serious crime for nearly 20 years.

Five properties across Auckland, including the gang's headquarters, linked to Doyle have been restrained by orders granted by the High Court in September after the police made an application under the Criminal Proceeds Recovery Act.

Justice Venning noted Doyle was unable to challenge the evidence presented by the police at the time of his ruling, which alleges Doyle and his former partner benefited from "significant criminal activity".

Doyle strongly denied the allegations, according to his defence lawyer Maria Pecotic who successfully opposed an application by the Herald to view the police evidence.

The addresses of the restrained homes, as well as the names of Doyle's children and grandchildren who live there, are suppressed.

Doyle is linked to six companies and trusts, including the Head Hunters' charity That Was Then, This is Now, which own four of the restrained properties.

As part of the police case, an investigation by the Ministry of Social Development alleges Doyle received $380,000 in social welfare payments over 23 years he was not entitled to.

About $19.9m was deposited into the bank accounts of the six entities associated with Doyle between 2002 and 2016 - half in cash - according to analysis by the Serious Fraud Office.

"Further there is evidence from two recent drug investigations conducted by the police that payments were made to the Head Hunters East Chapter, to senior patched members of that chapter, and in the case of one to Mr Doyle directly and/or the other interests associated with him," Justice Venning said.

Out of the total, $9.2m was deposited into the That Was Then, This Is Now charitable trust.

A "substantial number of cash deposits" were made by Head Hunters members and associates to the charity and labelled as "loans".

"Similarly, large sums of funds have been paid to Head Hunters members by the That Was Then This is Now Trust," Justice Venning wrote.

The judge said there also seemed to be a discrepancy of $2.4m between the trust's declared income and funds deposited into the bank account.

"There is an available inference the Trusts have been involved in money laundering," said Justice Venning, who noted the charitable status was under review.

The trust deed states the charity wants to be a bridge between prison and the community by integrating former inmates into society through fitness.

The gang recently won an appeal in an ongoing legal battle after the trust was deregistered as a charity.

The Charities Registration Board did not believe the trust was fulfilling its charitable purpose of rehabilitating prisoners, but agreed to reconsider the decision after an appeal to the High Court in April.

On the basis of the so-far uncontested evidence, Justice Venning was satisfied there were reasonable grounds to believe the properties linked to Doyle were acquired by fraud, criminal dishonesty, money laundering and other significant criminal activity.

These properties were restrained under the Criminal Proceeds Recovery Act which essentially forces someone to prove how an asset was paid for.

These cases are determined by the civil level of proof, the "balance of probabilities", rather than the much higher criminal evidential threshold of "beyond reasonable doubt".

"Police are committed to ensuring that people can not accrue wealth and assets as a result of criminal behaviour, at the expense of the safety of our community," said Detective Superintendent Iain Chapman at the time of the raids on Doyle.

Chapman also said police were looking for any information from the public or "professional bodies who are engaged in the financial affairs of the Head Hunters gang".

Operation Coin was the latest in a string of investigations into the Head Hunters across New Zealand over the last three years.

Guns, drugs, cars, cash - even stolen Star Wars figures - have been seized in Northland, Auckland, Tauranga, Rotorua, Wairarapa, Wellington, Nelson and as far south as Christchurch.

This has been a deliberate nationwide strategy to tackle the gang, which has aggressively expanded in numbers and geographic locations in recent years.

"The gangs were seen as controlling the methamphetamine market and getting stronger," said Chris Cahill, president of the Police Association.

"There has definitely been [a targeted] attack on the Head Hunters."