Some quick actions by the neighbours put out the small blaze as emergency crews helped the trapped man.
Weston said he was unsure how long the man was under the mower before they arrived.
"He was potentially under there for some time. We had to use a line rescue to pull him out and put him on a stretcher as he was quite banged up."
Weston said it was a multi-agency effort, with Police, St John and Wanganui Towing all involved.
"It put all our skills to the test."
A St John spokeswoman said two ambulances were sent to the scene, with the man being taken to Whanganui Hospital in a serious condition.
He was transferred to Wellington Hospital on Monday night in a critical condition.