A man who stepped in to stop a man running off with thousands of dollars of jewellery from am Auckland shop has dozens of stitches in his face after being hit with a hammer.

A 22-year-old man was later arrested at the Sylia Park shopping centre and will appear in court tomorrow charged with aggravated wounding and shoplifting.

Detective Sergeant Nick Poland says had it not been for the injured man's intervention, it's possible the offender would have left the scene with nearly $40,000 worth of jewellery.

He says after the man was hit, others took over the chase - with six people detaining the accused until police arrived.