Mrs Field said she walked in the reserve every morning and was disgusted someone would set a trap there.
"This is such a popular place with children," she said.
"They play here in the weekend - that's the concern for me.
"A lot of dogs walk here in the weekend."
She said she recognised the trap immediately.
"They are disgusting things - not only catching possums but anything."
She was relieved the animal had been found.
"I would have hated to think of it suffering."
A Masterton District Council spokesman said they regularly set humane traps in the reserve to catch possums.
A person who lays a gin trap could be prosecuted under the Animal Welfare Act, with penalties of up to three years' imprisonment or a fine up to $75,000.