Firefighters and the elite police tactical response group also responded to the emergency call after the pipe bomb was found in Palmerston about 1.30pm on Monday.
"In the process of doing the mowing they've come across a cylindrical object in the grass," acting superintendent Shaun Gill told reporters on Tuesday.
A police bomb technician inspected the device and found it was a home-made explosive device in a 15 to 20cm long pipe.
"They're designed to function and spread a lot of metal and fragmentation, so anything in a close radius can potentially be in danger," Supt Gill said.
The device couldn't be moved and the bomb technician was forced to neutralise it in the park.
This was done by triggering an explosion next to the bomb so it could no longer function.
"There was no immediate properties in threat," Supt Gill said of the pipe bomb.
"(But) the thought of a child picking this up is catastrophic to think about."
Supt Gill said pipe bombs were easy to make but capable of killing, maiming and destroying.
"So anyone playing with this is not playing with a firecracker. These aren't toys," he said.
"The outcome of a pipe bomb is that someone can get seriously hurt."
It is not known how long the device had been in the grass.
A police investigation continues.
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