Health leaders have penned an open letter pleading for the urgent opening of a supervised injecting room in Melbourne's CBD after "countless" overdoses.

In the letter addressed to Daniel Andrews and published before he announced his resignation as premier on Tuesday, the leaders called on him to deliver on his 2020 promise to establish Victoria's second overdose prevention service.

The plan for the CBD-based room remains in limbo after a Salvation Army hub on Bourke Street was flagged as a potential site in July.

"Since your announcement more than three years ago, unsafe injecting continues on the streets of Melbourne and drug harms in the City of Melbourne have worsened," the letter said.

The letter was signed by leaders of organisations including the Australian Medical Association, Royal Australian College of General Practitioners and not-for-profit Cohealth.

"In the last three years, more than 40 people have lost their lives to drug overdoses in the CBD, and countless others have suffered non-fatal overdoses in city streets," it said.

"There have been more than a thousand heroin-related ambulance callouts in the CBD."

A medically supervised injecting room trial in the CBD has been under consideration for several years, after an initial site in North Richmond was opened in 2018.

Overdose prevention was "basic healthcare" and the evidence was clear it saved lives, the leaders said in the letter.

They commended the premier for his promise to set up a supervised injecting room in Melbourne's CBD, but said he stood at "the precipice of a significant decision".

Cohealth addiction medicine specialist Paul McCartney said he and his colleagues treated people with opioid use disorder each day, which helped to reduce mortality, crime and social dysfunction.

"The problem is that we can't provide that treatment to people who are dead," he told reporters.

Lisa Peterson, speaking at Baptist Place in the CBD, said she had previously used drugs in the laneway, which the community knew as a safe injecting space.

The laneway was fitted with a light, a sharps container and had access to fresh water, and outreach workers would frequently visit.

"Since there was talk of a safe injecting space, this has closed down, and now there is nowhere safe for people to go and use," Ms Peterson said.

Former Victorian police commissioner Ken Lay delivered his report on drug-taking patterns in the CBD to Mental Health Minister Gabrielle Williams in June.

It has yet to be released and the Victorian government says it is still considering its response.

The Lay report highlighted the matter's complexity and the government was taking time to get it right, a government spokeswoman said.

Following the premier's resignation, Cohealth chief executive Nicole Bartholomeusz said the organisation looked forward to working with the incoming premier to build on progress achieved under Mr Andrews' leadership.

Tuesday's open letter had the support of Keep Our City Alive, a group of CBD residents, business owners and community members, and the Victorian Greens.