"Oh my god, I just can't believe it," she said, in tears.
"If that's what has happened, if he's gone under there like that, I just can't believe it."
She said the boy was a "very, very clever little boy" and had a younger sister.
"He was full of joy and bounce."
She said his mother was in town at the time of the accident.
A witness said he saw the driver tell the kids to "get out of the way" of the tractor moments before the boy was hit.
"There was a bunch of kids running around by the tractor. The driver told them to 'get lost' but they kept playing around the tractor. Then he got run over."
The boy's uncle, who asked not to be named, said the "happy little guy's" birthday was approaching and he had spent his young life growing up in the Camberley community.
Hastings Mayor Lawrence Yule was at the scene and said council would be working with police on a number of theories during an investigation to determine the cause of the accident.
"It's an absolute tragedy. A mum and dad's son has been taken away from them."
He said the council tractor driver was "very, very shaken up".
"We will investigate what happened and see what things need to be done to prevent something like this ever happening again.
"I will be making contact with the boy's family. Obviously this is the worst possible thing that could ever happen to a family and the parents of the young boy."
Mr Yule said the driver and his wife would also be offered counselling and support.
"It's a tragedy for the boy, his family, the driver, the driver's family and council staff.
"It's a residential playground and it's in the middle of the school holidays - it's just an absolute tragedy."
Mr Yule understood the driver had exited the tractor to tell the kids to leave the area where the machine was mowing before the boy was run over.
A local kaumatua later blessed the site, as family members and friends stood in disbelief, before police began their investigations at the scene.
Sergeant Ray Kirkby said last night police were continuing to examine the accident and finish interviewing all the relevant witnesses.
"We are in the process of gathering all the information and physically reviewing the scene.
"Tomorrow we will go over it all and try to determine how this tragedy has occurred."
Mr Kirkby declined to comment on whether charges would be laid against the driver.
The Police Serious Crash Unit is investigating the accident. A council investigation will also be launched, alongside a Ministry of Business, Innovation and Employment Labour Department investigation.