He said it appears the mower then landed on its side on top of the woman, pinning her underneath.
Mr Andrew said the large mower was lifted from the creek using a tow truck winch.
Police will be investigating the death on behalf of the coroner.
No further details of the circumstances of the death or the identity of the victim were available, with the name likely to be released today after next of kin had been informed.
National health and safety body WorkSafe NZ had not been notified about the death by edition time yesterday.
"Of course that doesn't mean we won't be at some stage - but so far we have not been notified and so have no knowledge or involvement," a WorkSafe spokesman said.
It is the second fatal lawnmower accident in just over a week.
Peter Ross Bailey, known as Ross Bailey, died when he was pinned under his ride-on lawnmower in a small creek on his rural Taranaki property on October 25.
- Additional reporting: Northern Advocate