AMBULANCE crews are being forced to care for a soaring backlog of patients in hospital loading areas and corridors because overcrowded emergency departments cannot take over.

Internal Health Department figures obtained by the Herald show that in recent weeks several of Sydney's largest hospitals accepted only about half of the ambulance patients taken there within 30 minutes - far short of the recommended 90 per cent.

Prince of Wales, Blacktown, Westmead, Royal North Shore and Sutherland hospitals and the Calvary Mater in Newcastle were the worst affected in May and June.

In one week, Blacktown took over the care of only a third of its ambulance patients within 30 minutes, taking ambulances off the road because paramedics must wait with people they have transported.

The figures present a grim portrait of NSW emergency departments as the states negotiate emergency performance targets with the federal government. Meeting these would attract bonus payments under health reforms agreed by the Council of Australian Governments.