She was on duty with other lifeguards when the call came in about 2.30pm.
A local man had taken his boy fishing in the morning, but they had become cut off by 2.5m waves smashing against the headland.
Ms Lillas and fellow lifeguard Barry Cutfield set off into choppy seas in an IRB, and could soon see the pair stranded on an elevated escarpment.
It was unclear whether they were injured, so Mr Cutfield powered the IRB as fast as he could.
When a small lull came between sets of large waves, Mr Cutfield and Ms Lillas saw an opportunity to rescue the pair.
But steady rain had made the rocks slippery.
"The thing that stuck in my mind," Mr Cutfield said, "was that we could see it was a hazardous job that she had to do ... and I certainly gave her the option.
"I told her if you don't want to do this, it'll be fine. And she did - I thought she was very brave."
He had not seen such a deed in his 12 years of lifesaving.
Mr Cutfield was able to edge the nose of the IRB close enough to the ledge for Ms Lillas to step off.
She then dashed bare-footed across the exposed rock in 30 seconds. Her tough feet had made the job easier, she joked.
She said she wasn't really nervous at the time, "but looking back afterwards, I think that things could have ended differently".
When she reached the pair, she used the man's cellphone to communicate with police co-ordinating the rescue from the surf club.
By the end of the phone call, the heightened waves had made it too dangerous to make a safe extraction by IRB.
Ms Lillas stayed with the 8-year-old for an hour, sheltering him from the wind and keeping him calm before they were forced to higher ground.
"His dad was saying he stopped crying when he saw our IRB."
She said the boy had wanted to go motorbiking instead of fishing that day.
"I was talking to him about Christmas, and that I've got a brother the same age as him ... that sort of thing - I just wanted to take his mind off everything."
All three were eventually winched into the Tauranga-based TrustPower TECT rescue helicopter and taken back to shore.
A crowd that had gathered, among them Whakatane Mayor Tony Bonne, met them with applause as they stepped out of the helicopter.
"I was so hyped up and, after everything, I was a bit overwhelmed," said Ms Lillas, who spent the night celebrating with friends.
"Thinking about it, there wasn't much we could have done differently."
She was thanked by the boy's father and has been described as a hero by local emergency services.
But she didn't consider herself courageous.
"I don't mind helping out where I can. I'm not usually the one who does that sort of thing, but you volunteer to do it. It doesn't happen often, but when it does ... you just do it."
