He had not been able to publicise the theft until he contacted Mr Horsley, who was overseas when the theft happened.
"We're both shattered and disappointed," he said.
"Nothing like this has ever happened in my gallery before."
Mr Milbank said he would review his security - currently he has a buzzer that operates when someone walks in the gallery door, but he said it did not always work.
"I'm looking at getting cameras and so on."
Mr Milbank said the best outcome would be for the sculpture to be returned - no questions asked.
Failing that, anyone with information or who knows where the sculpture is should contact Whanganui Police or call/text Mr Milbank on 027 6286877.