The office of the Māori King has been raided by Serious Fraud Office investigators probing claims of financial mismanagement.

The move was welcomed by senior Tainui figure Tukoroirangi Morgan who said "I'm overjoyed the SFO have gone in. They at least will have a level of competency to investigate a very serious allegation and will get to the bottom of it."

Morgan said the elevation of the case to the SFO, from an initial complaint filed with the Charities Service, marked a serious escalation.

"The fact that it is now in the domain of the SFO raises the seriousness of the allegation and I think that for tribal members, as a tribal member, I hope that it's dealt with swiftly," he said.

"Until the lid is lifted, and we get a sense of the scale and the seriousness of the allegations, and then have brought before the courts, only then will we be secure and confident on how our tribal money is being spent."

The raid this morning at the King's offices at the Endowed College Buildings in Ngāruawāhia follows a referral from the Charities Service who have spent the past year looking into claims of misspending at Ururangi Trust.

That Charities Service investigation, probing complaints made about the spending and actions of one individual at Ururgangi, effectively ended in June when the trust voluntarily deregistered.

"In 2017 the Department began an investigation under the Charities Act which resulted in the Trust being referred to the SFO. The Trust has voluntarily deregistered from the Charities Register, effective 1 June 2018. The Department is unable to provide further comment as this is an active SFO investigation," a spokesperson for Internal Affairs told the Herald.

A spokesman for the SFO confirmed the serving of search warrants this morning, and said they related to "an investigation concerning the Ururangi Trust".

"This included a warrant for the premises occupied by the Ururangi Trust. The SFO does not disclose the detail of ongoing investigations to protect the integrity of the investigation and those involved," the spokesperson said.

Ururangi is a registered charity intended to support the functions of King Tūheitia Paki, funded annually by Tainui to the tune of $2m. In 2015 it was the subject of an adverse Charities Service investigation and warned further issues could see its charitable status revoked.

That 2015 report, obtained by the Herald, detailed the King's $350,000 annual salary and raised concerns about 114 transactions between 2012 and 2014 totalling $120,691, relating to the purchase of jewellery, clothing and beauty treatments and almost $90,000 in cash withdrawals.

Ururangi has been the subject of a long-running Herald investigation canvassing lavish spending on luxury automobiles, international travel and a mysterious $46,000 invoice for stomach-stapling surgery paid by Tainui despite documentation being unclear as to who actually received the operation.

According to its governing legislation, to open an investigation the SFO director needs to "suspect that an investigation into the affairs of any person may disclose serious or complex fraud".

Financial statements filed with the charities register show the a company owned by Ururangi's executive manager Rangi Whakaruru used charitable funds to cover $83,000 of private travel - an sum turned into a low-interest related party loan that, according to the most recent accounts, had not been paid back.

In April Whakaruru, who had gastric band surgery in 2016, denied his operation was covered by the mysterious $46,000 invoice submitted to Tainui and paid out of a fund intended to cover the medical expenses of the King.