A sixth person has died in the Australian city of Melbourne after a storm triggered thousands of pollen allergy asthma attacks, the Victorian state government said on Sunday.

Heavy rain and winds on Monday, just as spring peaks in the southern state of Victoria, allowed rye grass pollen to absorb moisture and burst into smaller particles that sent the pollen count soaring.

"This was a tragic and unforeseen event and hospitals are continuing to treat 12 people for a variety of respiratory and other related conditions, in addition to those in ICU care," a Victorian health department spokesman said in a statement.

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Five patients are still receiving specialist intensive care treatment in hospitals and three people are in a critical condition, the spokesman said.

Across Monday and Tuesday, hospitals treated more than 8,500 patients, according to the Victorian health department.