Col Lucas was walking his son's kelpie 'Ozwald' with his partner Kellie on earlier this month when the dog "basically started going nuts" at a tree nearby.
The Wallabi Point resident said they were enjoying an ordinary walk near their home on mid-north coast of NSW.
"We just stopped and thought that there must be a goanna up the tree because he never barks like that," tMr Lucas said.
The couple carried on with their walk down to Wallabi Beach before returning to the spot 30 minutes later to see the tree had fallen over.
It really came to a shock to Mr Lucas that the exact tree the dog was barking at had "just collapsed," he said.
"It was a decent sized tree. If it landed on you, you were dead."
The tree looked to be healthy and didn't have any visible signs of decay, Mr Lucas said.
Wallabi Point was affected by the bushfires in late 2019, and sometimes trees that look fine could be a danger to the area, he said.
The owner of the dog is Mr Lucas' son, who is currently in hotel quarantine after spending 80 odd days sailing back to Australia from East Africa.
"I knew he was a clever dog but even that has got me rattled," said Ozwald's owner, Zack Lucas.