Saturday, 3 March 2018:

At just 17, Ariarne Titmus has pulled off a rare treble by clinching the national 800m freestyle title in a personal best time at the Gold Coast Commonwealth Games swim trials.

But it still wasn't good enough for hard marking coach Dean Boxall.

Titmus became the first woman in 14 years to nab the 200m-400m-800m national crowns at a meet and only the sixth woman in Australia's rich swimming history.

Yet Boxall wasn't happy with the final moments of Titmus' stirring 800m victory at the Gold Coast Aquatic Centre on Friday night.

He spotted her committing one of elite swimming's "no nos", slowing down to take a breath at the flags five metres from the finish.

She clocked eight minutes, 20.08 seconds to take three seconds off her personal best time and move to second on Australia's all-time 800m list.

But Boxall wasn't impressed.

"As soon as I finished the race, I saw Dean say to me 'you breathed at the flags'," Titmus said.

"(But) I'm really happy I'm on the team now and I get to do those three swims.

"I worked hard for this, so I'm pretty happy."

Titmus became the first woman since Linda Mackenzie in 2004 to pull off the treble.

Only dual Olympian Jessica Ashwood - who claimed 800m silver on Friday night - has gone faster over 16 laps.

But even Titmus admitted her 800m win was "bittersweet".

She had hoped to break the 8:20 mark and move within sight of Ashwood's national mark of 8:18.14 in the final.

"It's bittersweet," she said.

"I suppose I was pretty close to the (Australian) record, I thought I'd be a bit closer.

"I obviously have some things to work on leading into the Games, so it's something I can look forward to."

Titmus could even see the bright side of Boxall's tough love.

"I've got something to work on in each race," she said.

"It's better being that way than going into the Games thinking 'how am I going to get faster'."

AAP