The Queensland Ambulance Service (QAS) has been accused of botching an investigation into a drug-tampering scandal by ignoring evidence from a paramedic turned whistleblower.

The QAS also provided incomplete information to the Health Ombudsman who subsequently dismissed a complaint from an elderly patient who tried to raise the alarm.

Barbara Cook, 74, claims she was injected with water instead of the painkiller fentanyl while being treated for a broken leg in Brisbane's north in May, 2013.

Despite apparently receiving the maximum dose of 150 micrograms, she experienced no relief.

"I did a lot of screaming," Ms Cook told Background Briefing and 7.30.

"It was almost a primordial thing. I couldn't control the screaming."

Given the potency of fentanyl, Professor Kate Leslie, an anaesthetist at the Royal Melbourne Hospital, said any patient in Ms Cook's situation should have felt something.

"It would be unusual for there to be no effect if a sufficient quantity had been given," she said.

Tampering evidence dismissed as 'faulty manufacturing'

Months later, Ms Cook developed an infection caused by bacteria called pseudomonas aeruginosa, which is commonly found in water.

"It can become life-threatening," said Professor Peter Collignon, an infectious diseases physician and microbiologist at Canberra Hospital.

"It's much more difficult to treat because it's resistant to most of the antibiotics we've got."

Samford is one of the ambulance stations implicated in the drug tampering scandal. ( ABC News: Tim Leslie )

Ms Cook's son-in-law, Tim Watkins, who had been working as a paramedic, was suspicious.

Mr Watkins had previously uncovered evidence of drug tampering in the Metro North region.

He photographed a broken morphine ampoule that had been refilled with water or saline and glued back together.

When he reported this to his managers, they concluded "faulty manufacturing" was to blame.

In August 2013, Mr Watkins found more tampered ampoules in his drug bag while on the road.

The discovery resulted in a drug bust.

"I know a significant amount of narcotics were found to have been tampered with," Mr Watkins said in an interview.

Recordings show paramedic tried to raise the alarm

According to an analysis of QAS records obtained by Background Briefing and 7.30, potentially thousands of patients who were administered narcotics for pain relief in the region over a two-year period were at risk of being exposed to a tampered drug.

When Ms Cook reported her concerns to the ombudsman, her complaint was not upheld, partly on the basis that her timeline conflicted with that of the QAS.

Barbara Cook is disappointed with the response from the Queensland Ambulance Service. ( ABC News: Tim Leslie )

The QAS submitted that drug tampering had only occurred in the region over a four-week period in July to August, 2013, and that Ms Cook was treated by paramedics before then.

But audio recordings supplied to Background Briefing and 7.30 reveal the QAS had been given evidence suggesting the problem dated back to 2012.

"My view is the investigation was based on a false premise," Ms Cook said.

In a meeting with a QAS manager, Ms Cook's son-in-law, Mr Watkins, said he had proof drug tampering was happening much earlier than his superiors realised.

"It was about 14 months ago when this was first noticed," he can be heard saying in one recording.

But two years later, the manager said he had no memory of Mr Watkins ever making this claim.

"I don't recall that," the manager said.

The QAS also overlooked the testimony of the paramedic who was caught stealing and tampering with painkillers around the same time as Ms Cook alleges she was mistakenly injected with water.

The paramedic, who asked to remain anonymous, pled guilty in the Brisbane Magistrates Court to a string of drug tampering offences beginning in October, 2011.

When asked how the QAS could have missed this crucial piece of evidence, medical director Stephen Rashford did not offer an explanation.

"All I can tell you is that I wasn't made aware of that," he said.

Patients have 'no idea' they were put at risk of exposure

Barbara Cook also complained to the Queensland Health Ombudsman. ( ABC News: Tim Leslie )

Dr Rashford denied the QAS intentionally misled the ombudsman during its investigation into the complaint from Ms Cook.

"The connotation is we intentionally tried to cover this up, we would never do that," he said.

Dr Rashford did, however, acknowledge shortcomings made by staff who were tasked with getting to the bottom of the unfolding public health crisis.

"There is the old saying that 'if it comes down to conspiracy or incompetence, in general, it's incompetence rather than conspiracy'," he said.

"They're not incompetent people, they just made the wrong decision."

Dr Rashford admitted little had been done to notify patients who may have been put at risk of being exposed to a tampered drug.

"We wouldn't be able to actually even trace who might have received it," he said.

The QAS maintains the risk to the public was minimal and Dr Rashford said tighter drug security policies, including the use of CCTV cameras near drug safes, have since been introduced.

After trying for years to resolve the issue from within, Mr Watkins, who now lives in the United States, decided to go public with his concerns.

"It's horrible to imagine what's happened or what could have happened to these patients," he said.

Ms Cook is also speaking out in the hope of protecting others from a similar fate.

"A lot of other people still have no idea whatsoever that they were exposed to harm merely by being treated for pain in an ambulance on the way to hospital."