threats against car dealer Terry Mullens and the firebombing of his luxury Bentley as he prepared to testify against five Hells Angels bikies in court.

Terry Mullens, in Five Dock this week, has allegedly been the victim of threats. Picture: Tim Hunter 

POLICE are investigating serious threats against car dealer Terry Mullens and the firebombing of his luxury Bentley as he prepared to testify against five Hells Angels bikies in court.

The star witness in an extortion trial, police believe Mr Mullens was the alleged target of at least two attacks on his business at Five Dock in March.

Detectives ramped up his protection and increased patrols of his $15 million inner Sydney home -- other covert methods were also used to keep his family safe.

The five men set to go on trial had been charged with attempting to extort money from Mr Mullens in July last year. Prosecutors dropped the charges about a week ago after Mr Mullens' changed his mind about giving evidence.

The court heard "recent developments" were behind the withdrawal of the charges.

The Sunday Telegraph can reveal those "developments" include an incident on March 1 where Mr Mullens' Bentley was gutted by an explosion at his Five Dock caryard. Another three cars had windows smashed and petrol poured in them on the same night, however the accelerant did not ignite.

In a separate incident, a further eight cars were trashed the next evening -- in total, 12 prestige cars were ruined.

Two sworn police affidavits -- signed by Gangs Squad detectives -- will be tendered at BurwoodLocal Court on May 30 during a costs hearing.

One of the documents will be made public while the other will remain sealed with the magistrate.

The Sunday Telegraph approached the racehorse owner to discuss the matters but he refused to comment.

Those behind both of the alleged threats and attacks on Mr Mullens remain at large.

There is no suggestion any of the five men -- who were charged with attempting to extort Mr Mullens -- were behind any of the incidents.

The charges against the men followed an armed hold-up at Mr Mullens' Burwood caryard last July, revealed by The Sunday Telegraph.

During the incident eight men stole four high-performance cars and terrified staff with guns and knives. In a bizarre twist, the cars were returned, in pristine condition, and left outside the Paramatta Road showroom about 26 hours later.

Days later five men were arrested and charged with extortion, however the robbery remains unsolved.

Lawyers for at least three men are seeking legal costs against police, claiming they failed to "properly investigate the credibility" of Mr Mullens. Lawyer Paul Hardin told Burwood Local Court Mr Mullens' credibility was going to be the subject of a "vigorous attack".

Police dispute this and will tender the affidavits to support their argument.