A sixth person has died after a storm sparked thousands of pollen allergy-related asthma attacks in one of Australia's major cities.

More than 8,500 people were treated across hospitals in Melbourne after suffering asthma attacks triggered by adverse weather.

Six people have died so far as a result of their reactions, while five are receiving specialist treatment and three remain in intensive care, according to the Victoria state government.

Heavy rain and winds last Monday allowed rye grass pollen to absorb moisture and burst into smaller particles - allowing the pollen count to soar.

The state government said the outbreak had not been foreseen.

Last week, hospitals across Australia's second most-populous city were swamped with people requiring treatment.

"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.

Asthma is a common long-term inflammatory disease of the airways of the lungs - symptoms of which include shortness of breath and a tight chest.