The theft of between as many as 50 hay bales from the West Harbour Pony Club at Sawyers Bay is being investigated by Port Chalmers police.
Club members noticed the bales missing just before the weekend, vice-president Marie Dewar said.
The bales were stored in an unlocked farm shed on the club's Sawyers Bay property.
Mrs Dewar said they must have been taken by someone who knew where the hay was stored.
It probably happened under the cover of darkness, she said.
"It's devastating for our members. We are not wealthy people and the bales are going to cost between $7 and $10 each to replace," she said.
It was the first time the club had been targeted by thieves, Mrs Dewar said.
She urged people in the area to keep an eye out and report any suspicious behaviour, or information about the theft, to Port Chalmers police.