The paramedics' union says ambulances are being forced to wait up to seven hours at Sydney hospital bays with COVID-19 positive patients on board.

COVID-19 positive patients are being kept waiting for hours in ambulances as under-stress Sydney hospitals struggle to cope with demand, the Australian Paramedics Association says.

"We are at a point of time where 18 months into this pandemic, we are still struggling with getting patients into emergency departments," APA NSW president Christopher Kastelan says.

Numerous ambulance crews over the weekend were "stuck up to five, seven hours at a time", he told ABC TV on Monday.

"It means that paramedics are doing 16, 17 hour shifts without a break. For us, that is a concern for patients and paramedic safety," he told ABC TV on Monday.

"Waiting on the back of an ambulance for six hours when you are a COVID-19 positive patient and you are struggling to breathe, it is not ideal and we have seen the strain coming up for a period of time now."

The knock-on effect was that ambulances were unavailable for many hours with resources being called in to Sydney from the Southern Highlands, the Illawarra, Blue Mountains, Central Coast and Newcastle.

Some crews had responded to triple zero calls 100km away over the weekend, he said.

"It's only a matter of time that something significant is going to happen and that is going to be a poor patient outcome," he said.

Premier Gladys Berejiklian released modelling on Monday predicting COVID-19 case numbers will peak in the next week, putting the greatest pressure on hospitals in early to mid-October.

Mr Kastelan said the 750 paramedics injected into the system over the past three years had already proven insufficient to cope with the COVID crisis.

"Do not forget that normal medical and trauma calls are still happening on top of the COVID-19 concern," he said.