A long-time executive for Donald Trump is expected to be sent to New York’s notorious Rikers Island jail after being sentenced for helping engineer a 15-year tax fraud scheme at the former president’s real estate company.

Allen Weisselberg, the Trump Organisation’s former chief financial officer, pleaded guilty in August, admitting that from 2005 to 2017 he and other executives received bonuses and perks that saved the company and themselves money.

Weisselberg is expected to be sentenced to five months behind bars after paying almost $US2 million ($2.9 million) in taxes, penalties and interest and testifying at the criminal trial of the Trump Organisation, which was convicted on all counts it faced.

The sentence will be imposed by Justice Juan Merchan, who oversaw the trial in a New York state court in Manhattan.

Weisselberg would likely serve 100 days, with time off for good behaviour.

Those days will probably not be easy for Weisselberg, 75, at a jail known for violence, drugs and corruption. Nineteen inmates there died last year.

“You’re going into a byzantine black hole,” said Craig Rothfeld, a prison consultant helping Weisselberg prepare for lockup.

Many convicts in New York City facing one year or less behind bars head to Rikers Island, which lies between the New York City boroughs of Queens and the Bronx and houses more than 5900 inmates.

Mr Rothfeld spent more than five weeks at Rikers in 2015 and 2016 as part of an 18-month sentence for defrauding investors and tax authorities when he was chief executive of the now-defunct WJB Capital Group Inc.

He now runs Inside Outside Ltd, which advises people facing incarceration.

Another client is Harvey Weinstein, the former Hollywood movie producer convicted twice of rape.

Mr Rothfeld said he hopes Weisselberg will be segregated from the general population, and not placed in a dorm with inmates who might not know him but will know his boss, who is seeking the presidency in 2024.

“Certainly Mr Weisselberg’s 50-year relationship with the former president is on all our minds,” Mr Rothfeld said.

A spokesman for the city’s Department of Correction said the agency’s mission is “to create a safe and supportive environment for everyone who enters our custody”.

Rikers is scheduled to close in 2027.

Weisselberg was the star government witness against his employer.

He told jurors that Mr Trump signed bonus and tuition cheques and other documents at the heart of prosecutors’ case but was not in on the tax fraud scheme.

Though no longer CFO, Weisselberg remains on paid leave from the Trump Organisation.

He testified in November that he hoped to get a $US500,000 ($724,000) bonus this month.

Weisselberg testified that the company is paying his lawyers. It is paying Mr Rothfeld as well, a person familiar with the matter said.

Mr Rothfeld declined to comment.

Mr Trump was not charged and has denied wrongdoing. The Manhattan District Attorney’s office is still investigating his business practices.

Merchan will also sentence the Trump Organization on Friday.

Penalties are limited to $US1.6 million ($2.3 million).

Weisselberg remains a defendant in New York Attorney General Letitia James’ $US250 million ($362 million) civil lawsuit alleging that Mr Trump and his company inflated asset values and Mr Trump’s net worth.

-Reuters