A young girl approached them wanting to play and then her older sister asked if she could give it a go.
Nikau said they were initially impressed by the sister's ability to hold her breath, but this soon turned to concern.
"We got a bit worried when she didn't come up and we went down under the water to see what she was doing," Nikau said.
"We tried to pull her up to the surface, but her hair was stuck in the suction grate. We tried really hard to pull her hair out of the grate.
"I got my hand inside the centre of the grate to try to untangle her hair and pull it out, my hand was really red, but it wasn't bleeding."
The boys could see the young girl attempting to push herself away from the grate, struggling to free her hair.
"She was trying really hard to get to the surface," Nikau said. "We saw all the bubbles come out of her mouth and then she fainted and went all floaty. I sent Arlo to get a lifeguard and kept trying to pull her hair out of the grate."
The lifeguard was unable to free the girl's hair manually and called for more lifeguards before using a knife to cut the girl free.
The lifeguards performed CPR and resuscitated her at the poolside. She spent a week in Whakatāne Hospital.
Arlo's dad, Trevor Pimlott, a trained paramedic for St John, attended the incident and told the boys how proud he was of their actions.
"Arlo's dad rang to see how I was later that evening and to tell me we saved this girl's life," Nikau said. "He said we were heroes. He said he was proud of us and we did the right thing."
Nikau's koro, fire and emergency volunteer Rik Tauroa, told him if they hadn't raised the alarm and kept their cool, the girl could have died.
"He said we were little heroes. I told him I was tired because I had tried really, really hard to pull her hair away from the grate.
"A few days later he rang to see how I was, and I said I was so happy she didn't die and was going to be all right."
In a statement, Whakatāne District Council's aquatics and recreation manager Andrew Smith said it was an unfortunate event that was under review.
"The grate, which covers a tube to the balancing tank, meets industry standards and is what you would find in aquatic centre swimming pools throughout New Zealand.
"The council has since commissioned an independent review of the incident to ensure that an event like this cannot happen again. The tube has been temporarily closed until the review is complete and a suitable solution is identified."
At a council meeting last week, it was confirmed that WorkSafe would not be investigating the incident.
-Whakatane Beacon