The British Medical Association says the plan to separate those with coronavirus symptoms from other patients "will save lives" in the long run.
The 14 centres, which are not testing facilities, are being opened in Northern Ireland in a bid to ensure other GP services are maintained during the pandemic.
Doctors were originally asked to volunteer for shifts in the centres but have now been sent a letter which implies it is part of their contract.
Each practice has been asked to cover one session per week per 1,000 patients - this means a practice with 10,000 patients will be expected to fill 10 shifts.
But the letter from Northern Ireland's Department of Health states that elements of the contract have been stood down and that each practice is "required" to transfer a portion of its capacity to the centres.
It advises doctors that the Department of Health is considering how "death in service benefits could be provided" in relation to COVID-19 work.
A group of GPs, speaking anonymously, questioned why centres of this nature were being established in Northern Ireland but not in the rest of the UK or in the Republic of Ireland.
One said: "The plan will not protect routine GP services because doctors will be coming and going between practices and COVID-19 centres several times a week.
"By forcing us to work in these centres, they are exposing us to viral overload. There is a much greater risk of death not just for us but for our families."
The GPs also have concerns about the availability and quality of personal protection equipment, despite commitments made in the letter. One claimed they had bought their own in a "farm shop".
But Tom Black, who chairs the BMA's GP Committee in Northern Ireland, said: "The centres are recommended by the World Health Organisation. England and Wales will follow us here because these centres will save lives.
"Yes, we have a contract but shifts in the centres will be voluntary. You can choose not to work there but why wouldn't you?"
He added: "I perfectly understand that we are putting ourselves in the front line but this is a pandemic and that is what's required."