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Investigators will head to the Kaipara Harbour today in a bid to find out what caused yesterday's fishing charter tragedy. The Transport Accident Investigation Commission (TAIC) confirmed last night it had opened an investigation into a "marine accident". Police said five people who were on board the vessel Francie were dead and last night three remained missing feared drowned. That made it potentially New Zealand's worst maritime tragedy since eight people died when the Easy Rider fishing boat sank in Foveaux Strait in 2012. TAIC last night said it was "tracking" the continuing search and rescue operation. Two investigators were expected to arrive this morning. "The Commission's inquiry will seek to explain the circumstances and causes of the accident, with a view to helping prevent similar accidents in the future," a spokesman said.

"The Commission is unable to confirm further details of this afternoon's events while the search and rescue operation continues." A spokesman for Maritime NZ said it was not appropriate for the organisation to comment, given the police response and the fact the investigation was still under way. But he encouraged anyone who may have information to make contact. "If people have maritime information they believe is important or have concerns they wish to raise, then please contact Maritime NZ. "We will be grateful for their call." The Kaipara Harbour features one of the toughest bar crossings in New Zealand. It is estimated as many as 110 ships have been involved in serious mishaps on the bar. Bars "are common around the country in areas such as harbours and rivers," an education page on Coastguard Northern Region's Facebook page says. "If you go boating and want to get out into open seas you usually have to navigate them and they can also be extremely dangerous. "Many experienced boaties have got into difficulty crossing a bar and each year, boats are damaged and people are killed or injured when attempts to navigate bar crossings go wrong." • Anyone who has concerns about the welfare of relatives in relation to this incident is asked to attend Helensville Police Station or to call 111 for assistance.