The former boss of the Health Services Union has pleaded guilty to dozens of crimes relating to dodgy dealings spanning years.

Former Australian Labor Party president Michael Williamson funnelled almost $1 million to businesses he had an interest in, including one registered to his own wife.

Williamson faces jail for a raft of offenses after an investigation was sparked last year by accusations from the HSU’s national secretary, Kathy Jackson.

Accusations ran the gamut of cliché union-heavyweight dishonesty, including claims of money laundering, handling the proceeds of crime and the misappropriation of union funds totalling hundreds of thousands of dollars.

Williamson appeared in Sydney's Downing Centre Local Court on Tuesday morning, where the prosecutor told Chief Magistrate Graham Henson that several offences had been folded into four formal charges that Williamson would plead guilty to.

Williamson admitted dumping nearly $340,000 into a business called Canme Services, which was registered in his wife’s name. He also defrauded the HSU out of $600,000 through a consulting company called Access Focus, whose inflated fees reportedly led to a big windfall for Williamson.

Williamson was caught in the middle of shredding condemning documents when he was confronted at the union offices in Sydney last year. He has pleaded guilty to recruiting others to help him destroy evidence in the case as well.

There will likely be more arrests in the case, police say.

Mr Williamson’s salary of $400,000 is among the highest in the union, only approached by the alleged earnings of five of his family members also employed by the HSU.

Australian Council of Trades Union (ACTU) president Ged Kearney says Williamson deserves the full extent of any punishment handed down.

“Defrauding union members of their money is something that the union movement cannot abide and will not stand for,” she said.

“These offences are very serious and we're very pleased that they will be dealt with properly by the criminal law.”

Williamson’s crimes robbed members of the HSU including hospital cleaners, assisting staff and orderlies – some of the lowest paid unionists in the country.