Diseases that disappeared during the pandemic would "be back for sure" now that people were interacting much more, Associate Professor Nathan Bartlett says.
"There will be increases in transmission of cold viruses that cause asthma attacks, COPD [chronic obstructive pulmonary disease] exacerbations, influenza illnesses, RSV [respiratory syncytial virus] transmission and bronchiolitis in kids and the elderly," he said.
He was concerned that people had become "desensitised" to COVID deaths and there appeared to be a widespread feeling that "we're out of the woods and we don't have to worry about this anymore".
"That's completely wrong. It's not over by any means."
Dr Bartlett said more familiar viruses and bugs that cause the common cold and other illnesses would return with uncertain consequences. These pathogens would be "circulating with COVID".
Immunity to these other viruses - which had not been circulating due to people isolating - would be "much lower than normal".
"In a lot of examples, having concurrent or subsequent viruses is not going to be good for you," he said.
The so-called "flurona" - a term coined to describe a person who has the flu and COVID-19 at the same time - is one example. "Flu in itself can cause serious disease and make people more susceptible to other infections," he said.
He urged people to get the flu vaccine, saying uptake of the annual shot had been "very slow". Some people may be tired of getting vaccines, but Dr Bartlett said "you'd be crazy not to get a flu shot". He also urged people to get a COVID-19 booster.
Federal data shows about 66 per cent of eligible people in NSW and 69 per cent across Australia have had a third COVID vaccination.
"Without that third shot, you've basically got no protection," he said.
He was concerned by recent reports that more people had already died in Australia from COVID this year, than the previous two years combined.
"There are really high numbers of COVID still around," he said. "We have a far more transmissible virus and far less interest in being vaccinated. It's crazy."
He said the relatively lower booster rates and waning immunity were "contributing significantly to the high levels of transmission we're seeing and the deaths associated with that".
We have a far more transmissible virus and far less interest in being vaccinated. It's crazy.
More than 5000 COVID-19 deaths have occurred in Australia this year and about 7300 since the pandemic began. About 200 people died in the past week alone, federal data shows.
"The vulnerable and elderly are disproportionately dying. They are the ones who pay the price for people not being boosted, not wearing masks when they probably should and thinking it's all over," Dr Bartlett said.
"We really need to try to get this death rate down a bit. Where are we going to be by the end of the year?"
He added that another variant "could pop up at any time, evade existing immunity and cause further disease".
While many were happy restrictions were lifted, Dr Bartlett feared "we can't afford to completely drop our guard". "We still need to be doing what is practical to mitigate the risks."
He said this year would be a "uniquely uncertain phase of the pandemic".
People were learning to live with COVID and "re-learning how to live with the other infectious diseases that have been all but absent in the previous two years".
"As we head into winter, we need to be doing what we can to limit the impact of all these viruses - an impact that is going to be hardest on the most vulnerable in our community."
Meanwhile, the National Asthma Council Australia has recommended that people with asthma get a fourth COVID-19 immunisation as soon as they are eligible to receive it.