There's a heightened risk of thunderstorm asthma due to a longer than usual grass pollen season in southern Australia.

Ten people died and emergency departments were overwhelmed during Victoria's last major thunderstorm asthma event in 2016 and conditions this year are proving difficult to predict.

It's caused by high amounts of grass pollen in the air combined with certain weather events like thunderstorms which then trigger an attack.

It can affect people with diagnosed or undiagnosed asthma and hayfever but health authorities say preventative medication goes a long way towards reducing risks.

The official pollen outlook for the year is average but Victoria's season started early in September for the first time in three decades and could last into January, longer than usual.

Victorian Chief Health Officer Clare Looker urged anyone who gets wheezy, breathless or has chest tightness to get checked out by a doctor as soon as possible because conditions can quickly change.

"There's an element of unpredictability about these events and so it's still critically important Victorians understand how to protect themselves," Dr Looker told reporters on Wednesday.

Edwin Lampugnani from Melbourne Pollen said the season was on track to be bigger than initially thought.

"This year we're seeing conditions we haven't previously seen and it's very difficult to make a prediction," he said.

The season also started early across south eastern Australia including in the ACT and NSW but Victoria was considered a hotspot and has the only official thunderstorm asthma warning service.

At least one in 10 Australians have asthma and twice as many have a grass pollen allergy or hayfever, according to Peter Wark from the National Asthma Council Australia.

While there are no official estimates of how many people go undiagnosed, he said only 20 per cent of those with confirmed asthma used preventers which was much lower than it should be.

"If we want to prevent catastrophes like 2016, we would go a long way in managing this chronic condition in the first place," Professor Wark said.

He urged people to get checked by a doctor before it was too late.

"If you have an allergy to grass pollen and you're exposed to that high load of allergens it's an immediate assault upon your immune system," he said.

"That causes a severe and sudden reaction which is very, very hard to treat.

"Once that's happened, it's much harder to reverse the situation than try and prevent it from getting to that point."