Investigators into the fatal capsize of the Francie in Kaipara Harbour say they want to gather as much information as possible from survivors and witnesses while the event remains fresh in the memory.

The Transport Accident Investigation Commission has doubled to four the number of its investigators working on the inquiry in Auckland and in the Kaipara area.

They are being supported by head office staff in Wellington.

Chief investigator Captain Tim Burfoot says the present focus is on collecting information from the three survivors, next of kin, police, members of the Kaipara boating community and the Coastguard.

He says the commission team is keen to gather as much information as soon as it can while memories remain fresh.

A great deal of information has already been collected "but it is still early days in the investigation".

The Francie, a charter fishing boat, capsized in treacherous conditions on Saturday.

The bodies of seven of the 11 men on board have been recovered. Another man, 56 -year-old Taulagi Afamasaga, remains missing and is presumed drowned.

Capt Burfoot said some debris washed up on local beaches had been collected by police and would be passed to the commission for analysis.

He said the Francie, a 12-metre steel launch, had not yet been found.

"The location of the wreck is currently unknown," he said.

"The commission will consider that aspect of the inquiry once all other evidence has been gathered and analysed."

Capt Burfoot said the gathering of evidence would continue for some time.

He said the commission had the ability to issue urgent safety recommendations at any stage if required.