An Australian mother has given birth to a baby boy weighing 5.9 kilograms (just under 13 pounds).

Mum Breanna Sykes thought her baby was going to be big, but even experienced doctors were surprised when Ziad Kadic was born - weighing in at the size of an average three-month-old.

Mum Breanna Sykes knew her baby would be big, but his size even surprised doctors. Photo / Facebook/7 News
Mum Breanna Sykes knew her baby would be big, but his size even surprised doctors. Photo / Facebook/7 News

"My waters broke, and I was fully dilated, but the pain was too much, she told Perth Now.

"I was having an anxiety attack so they had to put me under anaesthetic. When I woke up they told me I had a 13.2 pound (5.9kg) baby."

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The bonny boy, who was born just after 7pm on Saturday in Perth, is 57 cms long, and was instantly too big for the newborn clothes his parents had bought for him.

"One of the doctors said 'this is the biggest baby I've ever seen in my 15-year career'. It's pretty amazing," Sykes told Nine News.

The first time mum realised her baby was going to be big when doctors told her Ziad would have to be delivered by caesarean.

"They said he's just not going to fit, if I try and push the baby he would break my pelvis so I had to get a caesarean," she said.

"But, he's here safe and happy and that's what matters."

Baby Ziad was instantly too big for his newborn clothes. Photo / Facebook/Nine News Perth
Baby Ziad was instantly too big for his newborn clothes. Photo / Facebook/Nine News Perth

Since his arrival, a stream of stunned doctors and nurses have come by to see him.

"He's very heavy. He's a double newborn. I've only had him for two minutes and I've already got a dead arm," Sykes said.

According to Perth Now, Ziad is one of the largest babies ever to be born in Australia.

-nzherald.co.nz