The victims were identified as a group of Pacific Island friends who regularly fished together, and the captain of the vessel.
"Investigators are working to establish exactly what happened," police Inspector Mark Fergus told reporters.
"We will be assessing whether there is any culpability on the part of anyone involved."
There were 11 people on the boat, The Francie, when it sank at the entrance to Kaipara Harbour, north of Auckland, on Saturday afternoon.
Three survivors were rescued and seven bodies recovered, police said. Aerial and shoreline searches continued for the missing man, who is presumed dead.
Fergus said there were life jackets on board The Francie but it was not yet known if they were used.
The 12-metre (40-foot) Francie went down when trying to re-enter the harbour in waves of up to four metres (13 feet).
The coastguard raised the alarm after the vessel failed to make the expected radio contact when it crossed the sandbar into the harbour.
Spokesman Ray Burge told Fairfax Media that radio logs showed The Francie was the only vessel to cross the treacherous bar on both Friday and Saturday.
"What I can tell you is that our volunteers reported four metre swells" in the harbour, Burge said.
A former skipper of The Francie, who now operates another charter boat, told the New Zealand Herald conditions were "too rough" to have gone out on Saturday and he cancelled his sailing.
"I stayed home, I'm not stupid," the Herald quoted Rod Bridge as saying.
The owner of Kaipara Cruises told Radio New Zealand the bar could get very rough and he would not have attempted to cross it on Saturday.
"I can tell just from being on the land here it would have been horrible down there, I'd say huge seas, breaking seas, it would have been not a very nice place to be," Terry Somers said.
Police Inspector Willi Fanene said the passengers were all of Pacific Island descent from Tonga, Samoa and the Cook Islands and aged between 31 and 59.
It is understood that they chartered the boat about once a month.
The Francie, a fishing charter vessel thought to have got into trouble on the Kaipara Harbour
An investigation was under way in New Zealand Sunday to determine why a chartered fishing boat capsized, claiming at least seven lives in stormy seas that kept other vessels inshore.
