Three people remain in a critical condition one week after Melbourne's "thunderstorm asthma" emergency, which killed six people.

Twelve people continue to be treated in hospital as a result of the unprecedented health crisis last Monday.

“Five patients remain in intensive care, three in a critical condition, following this tragic, unforeseen event,” a spokesman for the Department of Health said in a statement today.

Yesterday the sixth death relating to the storm conditions was confirmed.

More than 8500 patients were treated for breathing difficulties across Monday and Tuesday last week.

READ MORE : Woman who died in Melbourne storm remembered by family

The storm event saw a high pollen count and thunderstorm conditions combine to produce tiny particles in the air that penetrated deep into people's airways, causing attacks for asthma sufferers.

The Inspector-General for Emergency Management has announced an investigation into the cause of the deaths.