Inland NSW had several June temperature records broken on Wednesday including Bathurst which shivered through minus 7.5 degrees Celsius and Scone which recorded a minimum temperature of minus 5.1C.
The mercury plummeted to minus 1.6C in Campbelltown in Sydney's southwest, making it the coldest morning there in five years.
No other records were broken on the morning of the winter solstice but it dropped to just minus 7.2C in Canberra, 1.9C in Hobart, 4.2C in Melbourne and 7.9C in Brisbane.
Temperatures are on track to remain below average on Thursday and Friday but it's set to warm up after that, according to Bureau of Meteorology Senior Meteorologist Miriam Bradbury.
"The last couple of mornings we've seen 4-8C below the average across large parts of the east coast, tomorrow morning we're kind of only looking at one to three degrees below average," Ms Bradbury told AAP.
She said warmer than average conditions were expected for the rest of winter.
Pet owners have been asked to bring animals indoors if possible with cold nights tipped to stretch into the weekend.
An alert from RSPCA South Australia stated rabbits and guinea pigs were particularly at risk because their body temperatures can drop dangerously low in cold weather.
"If animals must remain outdoors, solid and warm shelter that is able to withstand rain, hail and wind is vital for the animal's safety and comfort," the alert said.
"Extra bedding may be required to ensure animals stay warm, especially very young or elderly animals, or animals suffering from health conditions such as arthritis."
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