Cases of whooping cough, the flu and gastro have surged, with the spike in illness hitting children hard and adding "unprecedented" pressure on some hospitals.
The winter lurgy peak is still ahead of northern parts of Australia popular with holiday-makers, prompting a leading general practitioner to urge anyone who hasn't had a jab to roll up their sleeve.
More than 170,000 influenza cases have been confirmed so far in 2024, with 29 per cent of them in children under nine years of age.
Infections in 2024 are already 27 per cent higher than the first six months of 2023.
Those figures don't give a complete picture as many people do not get tested, according to Royal College of General Practitioners rural chair Michael Clements, who says cases are likely to climb as the winter virus peak hits warmer areas in August or September.
"We certainly are concerned that this is going to continue to increase and potentially overwhelm the hospital systems," the Townsville GP told AAP.
His comments echo NSW Health Minister Ryan Park, who attributed "unprecedented" demand on emergency departments in his state to a 30 per cent jump in flu cases, combined with fewer GPs working in the community.
Victoria's health department warns vaccine coverage of influenza remains "low" in all age groups including children, despite a sixfold increase in emergency department presentations due to the flu from April to June.
Dr Clements said there's likely no single cause of the jump in flu, but it could be due to lower herd immunity, previous COVID-19 infection which has lowered immunity, more international travel and people going to work while unwell.
Cases of pertussis, known as whooping cough, have already surged past pre-pandemic levels to more than 12,900 so far in 2024, which is four times the number of total infections in 2023.
More than 60 per cent of cases have been in children under 14, with NSW health warning of "unseasonably high" infections of whooping cough and pneumonia in school-aged children.
It can be deadly for babies, with Dr Clements noting many infants and adult patients with whooping cough have also tested positive to several other viruses at the same time like respiratory syncytial virus (RSV) or COVID-19.
"These are spikes, we certainly have to acknowledge that they are big increases on the last years," he said.
"What's hard to know is how to interpret it, so whether or not we're just interpreting a return to normal behaviour post COVID."
Cases of a particular gastro strain have also already tripled the 2023 total, surging to more than 11,700.
Cryptosporidiosis, a type of gastroenteritis caused by a parasite, is spread in swimming pools, and anyone with symptoms is advised to stay out of the water for at least four weeks as they could still be shedding the virus, which can survive chlorine.
Dr Clements is concerned about what he described as the "massive explosion" in preventable illness and said he has noticed fewer campaigns from the federal government encouraging people to get jabbed.
"We can't look into the crystal ball and say whether we're at the peak now or whether we'll have another one," he said.
"It's never too late to get a vaccine."
