A cafe worker has detailed the moment bystanders took down a knife-wielding attacker on Auckland's North Shore

Police say the local man is in custody at North Shore Hospital - after stabbing and moderately injuring four people at Murrays Bay.

He then fled on foot to Mairangi Bay, but was brought down by members of the public.

Montrose Cafe manager, Damien, says customers coming in told him what they saw.

He says a person in crutches tripped up the offender, then a construction worker pinned him down - before hitting him on the head with the crutches.

Police say there were no warning signs of the attack, and it was not planned.