A worker from a nearby cafe 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 apprehended by bystanders.

Montrose Cafe manager Damien told Heather du Plessis-Allan customers coming in filled him in on their witness accounts.

“A member of public in crutches tripped him up and a construction worker from the nearby construction site working of the waterway pinned him down and he got hit on the head with one of the crutches.”

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

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