Overdose deaths from prescription opioids are at record levels in Australia, a Sydney inquest investigating the deaths of six people has heard.

The woman and five men — who cannot be identified — died in their homes or in hotels in May 2016.

One person died in a hotel just metres away from the legal injecting room at Sydney's Kings Cross.

Counsel assisting Peggy Dwyer said alcohol was also found in all their systems and the inquest will examine other common themes in the deaths.

She said the inquest was an unusual one because most opioid overdose deaths are not treated as suspicious.

She said this cluster of deaths is part of a much bigger problem and the inquest will examine if there is a need to make Naloxone, which reverses opioid overdoses, more readily available.

The inquest heard around 750,000 people in Australia are dependent on opioids.

"Pharmaceutical opioid deaths in Australia now exceed heroin deaths by a significant margin…the reverse of what was seen in the 1990s," the counsel assisting said.

Between 2011 and 2015 there were 2,145 deaths associated with oxycodone, morphine, codeine, fentanyl, tramadol and pethidine, and 985 deaths due to heroin.

"According to a report released by the Therapeutic Goods Authority in January 2018, since the end of 2009, there has been a general increase in prescriptions in Australia including opiated prescriptions," Ms Dwyer told the court.

"Levels of prescription opioid overdose, including accidental overdose, are at record levels in Australia and internationally."

She noted that prescription opioid deaths dominate in the over-30 age group.

The problem with fentanyl

Ms Dwyer said the misuse of fentanyl — which is a potent synthetic opioid up to 100 times stronger than morphine — is a particular concern for police.

She said NSW Police had provided her with information that between March 2016 and February 2018, 26 per cent of all drug-related deaths were attributed to opiates or opiate-based derivatives, and 32.29 per cent of those were directly attributed to fentanyl.

Ms Dwyer said the illegal manufacturing of fentanyl in the US had led to a public health crisis.

US musician Prince overdosed after administering a drug containing fentanyl. ( Reuters: Jumanah El-Heloueh )

She referred to the death of singer-songwriter Prince, who took a counterfeit painkiller containing fentanyl.

She also noted that fentanyl abuse was a big problem in some parts of regional Australia where heroin was less readily available.

Deputy NSW coroner Harriet Grahame, who is investigating the six deaths, acknowledged the family members and friends who were sitting in the public gallery.

"I offer you my sincere personal condolences for the losses you have suffered," she said.

"A number of our fellow citizens are dying of opioid overdoses … too many of them.

"They are unnecessary deaths in the community."