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North Shore Police are investigating after blood was found in two public areas of Beach Haven, Detective Senior Sergeant Brett Batty said.
Blood was found on Beach Haven wharf about 3.30am, and another bloodied area on the footpath on the corner of Rangatira Rd and Beach Haven Rd.
The two locations are around 400 metres apart.
"A man is in custody and is assisting Police with inquiries," Mr Batty said.
The man was arrested near Beach Haven wharf on a disorder matter, but is not injured.
"We are working through several lines of enquiry but at this point in time we simply don't know what's happened," Mr Batty said.
"It could be that a fight has taken place and the other party hasn't phoned police or sought medical help, but whoever that blood has come from, we need to know."
Police have cordoned off the Beach Haven ferry terminal, a driveway on Beach Haven Rd as well as a bus stop just 30m from the Beach Haven Police Station.
Each crime scene is spattered with blood, as is the bonnet of a car outside Beach Mini Superette.
Forensic investigators in blue boiler suits have been inspecting each crime scene this morning.
Superette owner Param Patel said he had been told by police not to touch his car as they were taking fingerprints.
Police also checked CCTV footage from his dairy, as well as a liquor store where worker Daniel Philips said customers had reported many residential mail boxes being smashed overnight.
Residents on Beach Haven Rd said police had knocked on their door trying to identify what happened to the attack victim.
The residents, who asked not to be named because of fears that gang members were involved, said police told them a person had lost 2-3 litres of blood.
"We woke at 1am, heard a kerfuffle and a loud crash," one resident said.
"They sounded like big fellers, they had loud voices."
"I saw a crowd at the bus stop at 10.30pm. They were all wearing black," Mr Patel said. "I didn't hear any [unusual] noises, it's always noisy, always crazy here."
Forensic investigators were asking members of the public to keep off the blood trail.
Rangatira Rd resident Susy Oommen lives behind a cordoned-off bus stop with blood spattered on the ground. "I heard a sound around 11 last night, people talking, the sound of a toy being dragged," she said.
The Police Eagle Helicopter and the Police Boat Deodar have been searching the coastline this morning.
Police want to hear from anyone who has information that could assist.
"If you were in the Beach Haven area last night and heard or saw something that you think could help us, then please call the North Shore Police on 09 488 9754," Mr Batty said.