Michael Hill Jewellers is bringing in security guards at all 34 North Island stores to fend off robbers

Its chief executive says it follows recent, alarming offending in Waikato, Wellington and New Plymouth.

Daniel Bracken says it's an extension of security measures - which includes fog cannons and personal alarms for staff.

Police are still hunting two hammer-wielding men who robbed a New Plymouth Michael Hill on Friday.

Five people were arrested after an attack on a Michael Hill store in central Wellington, the following day.