Hospitals across Victoria have been left reeling after an unprecedented number of people suddenly fell acutely sick on Monday evening, in what is believed to be a mass incident of "thunderstorm asthma".

Ambulance Victoria was flooded with calls after a storm hit Melbourne shortly before 6pm, prompting a "major disaster response" from emergency services.

The demand was so great that Ambulance Victoria ran out of ambulances and had to call in police officers, non-emergency patient vehicles and field doctors trained for disasters to help with transporting acute patients to hospital.

State health commander Paul Holman said in his 40 years as a paramedic he had never seen an event like it.