An internal police audit found that drugs listed as destroyed have been kept, while chemicals that should have been stored are missing.

Senior police officers admitted privately that they are unable to say what exactly happened to the missing drugs.

"The truth is we will never know," said an anonymous police source.

"Many cases go back years and it is impossible to find out what really happened in each case."

Previous police audits of the forensic service have left the problem unresolved -- it is now up to the Ombudsman to investigate.

The Ombudsman's involvement began after allegations from within the police force that there was a serious problem with the handling and storage of drugs by the forensic service.

Ombudsman investigators took the allegations seriously enough to register a member of the police department as a protected internal source.

Investigators do have some leads relating to corruption cases dating back over 15 years ago.

In 1991, police discovered that 10 kilograms of seized chemical used to make amphetamines had been switched with tile grout after it had been seized.

Further investigations found that drug squad detective Kevin Hicks organised several burglaries to allow criminals to steal back seized chemicals.

He was later given a five year jail sentence after pleading guilty to theft, bribery and burglary charges.

A spokeswoman for the Ombudsman's Office refused to comment on the new inquiry.

"We cannot provide any information at all," she said.