An ultrasound on his mum Whitney Allison at 24 weeks into her pregnancy, revealed Marcus had Hypoplastic Left Heart Syndrome - half a heart - a condition that strikes one in one million.

It's a rare and little known of congenital defect which comes with a high mortality rate and termination is recommended.

For Whitney and Marcus' dad Troy Dunn, that wasn't an option. They wanted to give their boy a chance at life - that decision paid off.

"The options were to terminate the pregnancy, or deliver at term and hold our baby until he died in our arms, or deliver him and opt for high-risk surgery, which is what we did," Whitney said.

"We knew there was a low survival rate and doctors strongly recommended I terminate the pregnancy, but we were willing to take the risk."

Marcus underwent his first open heart procedure in Sydney's Westmead Children's Hospital when he was two days old. A shunt was inserted into an artery to maintain blood flow into his lungs.

He had a second surgery before returning home to Rutherford with his parents in August 2015. The family had stayed by Marcus's side at the hospital for the three months after their little boy was born.

Every hurdle this little battler has hit, he has fought off and come out stronger. "It has been very scary, particularly when he had bronchitis as a baby. He would stop breathing and was unresponsive," Whitney said.

"It was touch and go there for a while."

Marcus has continued to astound the country's leading paediatric heart surgeons with his grit, determination and courage.

Last week Marcus, now aged four, had his third and hopefully final round of surgery to improve his oxygen circulation. Whitney said this round was the toughest.

"It's been challenging," she said. "Marcus is older and has a voice which he's not afraid to use. It's been hard but he is as strong as ever.

"As a parent it has been hard listening to him cry in pain and have him tell you he's hurting. Having to hold him down while they poke and prod him with needles and have him say 'mum, stop it. It hurts mum. I hate you mum,' breaks your heart.

"You know you can't take his pain away and it's even harder because you know he doesn't understand why all this is going on," she said.

What was supposed to be a six-hour surgery was completed within four hours - doctors pleased with the result.

"Marcus is still recovering and has a chest drain in, but he's almost back to his normal self. Once his circulation gains momentum he'll be able to do what everyone else does. He can finally enjoy life.

"He'll be able to play sport but he won't be a 100 metre sprinter," she said.

"Doctors now believe Marcus can tackle life head on and live just like any other kid."