"To hear him talk was the most amazing thing ever!!!!!!!!!," Dawn posted on his GoFundMe page.
His tracheostomy tube had also been removed.
Josh's sister Sian has also flown to New Zealand to be with her family and is currently spending two weeks in MIQ before joining her family.
"We phoned Sian and when she answered she was stunned to hear Josh say hello. Needless to say a lot of happy tears have been shed today. I know we keep saying it but we have to remember he has still got such a long way to go."
Earlier in the day, he had also made progress by staying awake for most of the morning.
His parents saw him for the first time in three years last weekend after he had woken from his coma. He had only been able to make hand gestures and continued to sleep a lot.
At the time, Dawn said seeing her son smiling was "unbelievable" after so much time apart and due to the circumstances.
It comes after the family had a stressful battle with Immigration New Zealand over being granted border exemption to enter the country to be with their son.
The family earlier told the Herald the support from friends, family and strangers had helped them get through this tough time.
A GoFundMe page set up to help the family visit their son has so far raised £11,195 ($22,049).