Three Tauranga surfers have rescued three children from the same family after they were knocked over by a wave and swept out to sea.

Six children from the family were at the beach in Mount Maunganui when the wave struck, and four of them, aged nine, eight, seven and 12, got in difficulty about 2.50pm, Sergeant Craig Madden of police search and rescue said.

One girl called for help and a surfer on the beach rescued a girl, who was struggling in the breakers, on his surf board.

When he got her back he rescued another distressed child being dragged out in the rip, Mr Madden said.

The 12-year-old girl managed to get herself safely back to shore.

A girl said her brother was still missing and a man on a surfski and another on a surfboard went looking for him, Mr Madden said.

They found the boy struggling to stay afloat, about 350 metres offshore well behind the breakers. They could only see his face just above the water.

Police search and rescue staff and the Mount and Omanu surf clubs, also called to look for the boy, were relieved when they arrived, Mr Madden said.

"It was very likely we would be dealing with another drowning if it had not been for the actions of these three men," Mr Madden said. "This highlights the need to swim in the areas patrolled by our local surf clubs."

An adult family member was watching the six local children, aged between five and 12, when the wave struck.

The four children swept away were treated at Tauranga Hospital.