Asylum seekers and refugees are waiting as long as five years for specialist medical treatment even when it has been formally recommended by the Australian government’s contracted doctors, new data has revealed.

Support workers and medical groups have accused the government of maintaining a “dangerous and sometimes fatal” system of care on Manus Island and Nauru, as MPs prepare to vote on a medical transfers bill which could see up to 1,000 people access long-awaited treatment within weeks.

The analyses of patient data and letters to MPs calling for urgent and dramatic change come as Médecins Sans Frontières launch a telehealth service to resume treating the patients it was forced to leave behind when it was kicked off Nauru by the Nauruan government last year.

Aggregated data from the Asylum Seeker Resource Centre, seen by Guardian Australia, revealed dozens of people are still waiting for medical transfer years after it was recommended by doctors employed by the federal government’s contracted health provider, IHMS.

A medical transfer was recommended by at least one IHMS doctor for half of the sample, as they had been diagnosed with medical conditions that could not be treated on site and needed specialist care.

“Medical transfer requests are made sparsely and reluctantly because doctors know they will be blocked by the department,” a former IHMS doctor, Peter Young, said.

The data revealed waits of mostly two to three years, but included six people who have waited more than four years for transfer.

Two men on Manus Island, aged under 40, were first recommended for transfer more than five years ago. One suffers chronic migraines and headaches, epilepsy and blurred vision, and requires a neurosurgical or neurological consultation, the data sheet said.

The other requires orthopaedic treatment, according to the data sheet, and suffers an anterior cruciate ligament injury, arthritis and mental illness. The data, drawn from IHMS medical records of 50 of its 70 casework clients on Manus Island and Nauru, provided a snapshot of what was a far bigger problem, the ASRC said.

“There are over 1,080 people held offshore, and the ASRC has a waiting list of 215 people needing our help to push for desperately needed medical treatment,” said the ASRC detention advocacy manager, Natasha Blucher. “The crisis is bigger and more severe than the government is saying.”

All patients listed in the dataset remain on Manus Island and Nauru, the ASRC said.

The Manus Island records only continue until March 2018 – when IHMS left the island, but the ASRC said none of the patients had been transferred since Pacific International took over.

The data release comes as MPs prepare to vote on the migration amendment (urgent medical transfers) bill, which seeks to legislate expedited medical evacuations for refugees and asylum seekers to Australia with the recommendation of two doctors, and without government interference except in cases of national security.

The federal government has said allowing doctors to dictate transfers is a threat to national security, while proponents of the bill say it includes ministerial discretion to refuse a transfer on national security grounds.

On Thursday News Corp reported unsourced Asio advice that the bill would dismantle the “third pillar” of the government’s border control policy. It included the concession that up to 1,000 people would be expected to be transferred to Australia within weeks of the bill passing, once the current blocks were lifted.

In a letter to all federal MPs this week, Doctors for Refugees said the current barriers to an Australian standard of care raised the question of “whether the Australian government believes that a group of people under our custody are less deserving of medical care than others”.

“The Australian public has been repeatedly advised by politicians that care provided offshore is broadly comparable to care on mainland Australia, but there is an enormous and growing body of evidence which contradicts this notion.

“The factor which continues to have the most profound negative effect on provision of care in serious cases is the delay/prevention of medical transfers.”

Doctors for Refugees said it had reviewed more than 400 cases and found inherent factors including inadequate facilities, geographical isolation, poor communication between medical service providers, bureaucracy, and delays by the Australian Border Force, had led to a lack of adequate medical care.

The organisation said that mental illness was implicated in eight of the deaths in the offshore system since 2013.

On Thursday the international medical NGO, MSF, announced it would resume treating its patients on Nauru through a telehealth service, in order to address the continuing mental health crisis.

The psychological consultations will be available for all former patients who are still on Nauru, including refugees, asylum seekers and Nauruans.

“Since our forced departure in October, we continue to hear from patients who are experiencing severe levels of mental health distress and who struggle to access quality psychological care,” said Christine Rufener, a clinical psychologist and MSF mental health activity manager.

“This telehealth service is MSF’s only remaining option to continue to act on our medical and ethical commitment to former patients.”

MSF was engaged by the Nauruan government to deliver mental health services on the island from November 2017 and operated an open-door policy treating local Nauruans as well as the asylum seeker and refugee population sent there under Australia’s offshore processing system.

However in October the Nauruan government abruptly ordered the medical NGO to cease its services immediately and leave the country, accusing it of conspiring against it.

In return, MSF publicly called for the immediate evacuation of all refugees and asylum seekers from Nauru, detailing the extraordinarily high rates of mental illness.