Cameron Smith has shown his class - and a rare display of frustration - while seizing command of the NSW Open with another sublime round on the Murray River.

As his playing partner, LIV Golf teammate and halfway leader Lucas Herbert endured an extraordinary back-nine collapse, Smith surged to a two-shot lead entering Sunday's final round.

Golf's so-called moving day proved Smith's mantra as the 2022 British Open champion threatened to equal the course record he set on Thursday before settling for a five-under-par 66 at Murray Downs Golf and Country Club, near Swan Hill.

With a supreme driving display complementing his wizardry short game, Smith reeled off five birdies and an eagle to soar to 15 under and deliver a 'catch me if you Cam' challenge to chasing rivals.

"It was really solid. I didn't really do much wrong, to be honest," Smith said.

"Played smart when I had to and got the putter rolling a bit there, particularly on the front nine.

"Leading into the last day or being thereabouts is where you want to be.

"I just need to do more of what I did today."

"That was about it. It was pretty boring golf, but it was solid golf."

But it was a despairing day for Herbert, who watched a two-shot halfway lead disintegrate into a four-stroke deficit.

He was only one stroke behind before two errant drives led to double bogeys in four holes down the stretch.

Herbert flung his driver in disgust after losing his tee shot right into the trees on No.16.

But worse was to follow with an inexplicable four-putt from just a few metres off the green as Smith birdied in a second three-shot swing of the day.

If not for a double of his own on the 17th, after finding two bunkers and then three-putting and dishing his ball into the water in anger, Smith would have a four-shot advantage.

"It was really a shame to not finish as I wanted to," he said.

"I feel as though I did what I need to do today and on 17 there, it was just, I don't even know what it was.

"But, yeah, even though it was a great round, it's a bit frustrating not to be even better."

Instead the former world No.2 has a two-stroke buffer over the people's player Corey Lamb "Chops", who eked out a scrambling par at the last in a round of 67.

Herbert's round of 72 left the Bendigo-born star in a three-way tie for third at 11 under with New South Welshman Alexander Simpson and New Zealander Josh Geary (69).

The hunt for the Kel Nagle Cup looks a five-way battle, with Smith six shots clear of the best of the rest.