A New York judge ruled on Monday that former US President Donald Trump should be fined $10,000 a day as long as he refuses to provide accounting and tax documents as part of a civil investigation into his group, accusing Trump of contempt of court.

"This is a huge victory, as a court has ruled in our favour, convicting Donald Trump of obstruction," New York Democratic Attorney General Letitia James wrote.

The attorney general - who has been conducting an investigation of possible tax fraud in the Trump Organization for years - added that "Donald Trump has to pay $ 10,000 per day for every day he continues to defy a court order to turn over documents to my devices."

On February 17, James obtained a New York judge's order ordering Donald Trump, his son Donald Jr and his daughter Ivanka to testify under oath as part of this investigation, and the Trump family has appealed the decision.

The court also demanded that a series of accounting and tax documents be handed over to the Trump Organization before March 31.

In the absence of a response from the organization, James asked the New York State Supreme Court to hold Donald Trump responsible for "obstructing (the investigation) for his refusal to comply with a court order."

Trump stabbed

The New York attorney general suspects that the Trump Organization "fraudulently" overestimated the value of its real estate holdings when it requested bank loans, while understating its value with the IRS to pay lower taxes.

Trump's lawyer, Alina Haba, said he plans to appeal the contempt order, adding in a statement: "We respectfully reject the court's decision."

If Trump doesn't pay the fine, he could be jailed under New York state law that allows judges to jail people who don't respect contempt of court rulings, Sarah Krisoff, partner at Dai Pitney Law Firm, said.

But she added that this is not likely to happen, as the judge can order other decisions such as increasing the amount of the fine.

Parallel to this civil investigation, there is a separate criminal investigation in Manhattan into allegations of possible financial fraud within the Trump Group.