The drowning occurred at the normally sheltered Cable Bay, near Mangonui in Doubtless Bay, just after 4pm on Tuesday.
The children, all siblings or cousins, were caught in a rip at the western end of the beach.
The father called for help then tried to rescue them before getting into trouble himself.
He was unresponsive when a member of the public managed to get him to shore.
Mangonui firefighters and St John medics performed CPR for more than 30 minutes to no avail.
At least half a dozen people were involved in the rescue.
Mr Murdock said Cable Bay was not usually regarded as dangerous but on Tuesday the water was choppy with a strong onshore wind and a rip running at the western end.
He was due to meet some of the rescuers today to collect witness statements and thank them for their efforts.
The 11-year-old girl is now in a serious but stable condition at Whangarei Hospital.
The deceased's name has not been releeased.
-Northern Advocate