Hard knocker

Definition
A hard knocker is a bowler hat.

Examples in New Zealand literature
From Frank Sargeson's short story "Conversation with my Uncle."
 * "My uncle wears a hard knocker. His wife put him up to it."
 * "We don't want a world full of dead men walking about in hard knockers."

Source
Sargeson, Frank. "Conversation with my Uncle." A Man and His Wife/ Christchurch: The Caxton press, 1940. 39-40.