Suffering from an immune regulation dysfunction known as acute necrotising encephalopathy of childhood (Anec), Tobin was taken to intensive care and put on life support.
That night, paediatric neurologist Cynthia Sharpe administered steroids and an immune replacement therapy in a bid to give Tobin the best chance at a normal life if he survived.
She had already treated his older sister Lucia, 7, for three similar episodes brought on by viral illnesses several years earlier so she was able to recognise and understand Tobin's situation faster.
Only 250 cases of Anec are known of worldwide. It has a high mortality rate and those who survive usually suffer permanent brain damage.
The Picu team spent four days making sure Tobin was stable enough to allow his brain a chance to recover.
"We monitor children with brain injuries very closely to make sure they are getting enough oxygen and blood flow to the brain," Dr Beca said.
But after five days, Tobin remained comatose and his parents, Jared and Sarah Chisnall, decided to take their only son home.
Dr Beca said the transformation in Tobin between the last time he saw him and yesterday was astounding.
"When I last saw him he was in a coma and going home and his whole future was very uncertain. We really didn't know whether he would live or die and we didn't know if he would end up with a whole lot of brain damage or not.
"So it's fabulous just seeing him back here and doing so well."
Dr Sharpe said Tobin's recovery was "definitely a lot better" than she expected. "It's wonderful. I am absolutely delighted."
She said the uncertainty of medicine made cases like Tobin's impossible to predict and dealing with such unpredictable situations was "part of the art of medicine".
"Sometimes it's clear that a child is brain dead. That's very straightforward. [With] other injuries we've always got to counsel the family that there's a range of possibilities, from a very good outcome, such as Tobin's had, to moderately to severely disabled or death."
Mrs Chisnall said meeting the doctors and nurses again gave her a chance to say thank you.
"They went above and beyond. On a medical level they were excellent ... but on a personal level they were just fantastic. As horrendous as it was, it was probably our best hospital experience."
Tobin's disorder
• Tobin Chisnall suffers from acute necrotising encephalopathy of childhood (Anec), which affects the immune system.
• There are only 250 known cases of the disorder in the world, including Tobin and his older sister Lucia.
• Many sufferers are left with permanent brain damage.
