The chill comes off the back of relatively light winds and clear skies.
Canberra has woken to a chilly minimum of 1 degree, while Perth and Hobart both recorded 6 degrees.
The mercury plummeted to 8 degrees in both Sydney and Melbourne, while Brisbane, and Adelaide hit 9.
Temperatures were balmy in Darwin by comparision, which saw a low of 17C.
Records were broken in Pingelly and Munglinup in Western Australia, where residents woke up to 0.2 degrees, while Cessnock Airport in New South South Wales had their coldest June temperature since 2006 recording -1.2 degrees.
“Winter is going all-in this year, leaving no stone unturned,” Weatherzone said.
The first upcoming cold front will dump snow across the Alps today before another trailing front delivers some follow-up snow tomorrow.
Read Related Also: Slain paramedic's wife gives birth two months after alleged murder
The two systems combined should bring around 20 to 40cm of snow by Tuesday.
Lack of snowfall and warm weather puts Aussie ski season at risk
The warning was officially upgraded earlier this month from “watch” to “alert”.
Weatherzone warned the chilling mornings could become consistent if El Niño develops.
“Winter is settling in, and with the persistent influence of El Nino, our weather patterns are taking a particular twist, Weatherzone said.
“Under El Nino’s influence, we can expect drier and clearer skies, the ideal conditions for nighttime cooling. So, brace yourself for a parade of cold, foggy and frosty mornings in the months ahead.”
Sydney is expected to reach 19 degrees, Melbourne 15, Brisbane 24, Perth 17, Adelaide 16, Hobart 14, Canberra 10, and Darwin 32.