Adam O'Brien was left "shocked" by the latest disastrous swing in Newcastle's faltering defence, which has seen their scorelines yo-yo 88 points in the past three weeks.

The bitterly disappointed Knights coach said after a 42-12 shellacking by the Roosters that his side's efforts without the ball are "back where they started" a few weeks ago in a dismal loss to the Warriors.

After being hammered 36-6 by the Kiwi side in round 16, O'Brien was especially pleased with the sharp defensive turnaround in last week's 38-10 touch-up of Cronulla.

But after conceding more than 42 points to the premiers – which could have been more had some final passes stuck for O'Brien's old side – the Knights are left lamenting the most inconsistent form line of the finals-bound sides.

"It was a polar opposite to last week," O'Brien said.

Tuala gets the Knights first

"The dressing shed's disappointed. The execution of some our defence tonight was a real shock to me considering what we managed to do against the Sharks, who are very good attacking team.

"We got some lessons tonight. We came up against a very good footy team that's hungry for more success and they made us pay.

"I was shocked to see that. We've had those conversations a few times now.

"There's been a couple of other performances this year, where you think that [those defensive issues] are put to bed but clearly they've come back to bite us."

Shibasaki collects a Pearce kick

Newcastle's heavy loss to the Roosters leaves them in seventh spot on the ladder and facing an uphill battle to rise above South Sydney to jag a home semi-final.

They next face St George Illawarra at home for the club's old boys day next Sunday, with O'Brien at least expecting to name a full-strength side.

Kalyn Ponga and Hymel Hunt will both come back in after being rested, while Mitch Barnett is also expected to take his place after managing some niggling injuries.

O'Brien was blunt when asked if Ponga's absence at the back had impacted Newcastle's defensive cohesion.

"You don't need your fullback to miss tackles," he deadpanned, well aware how dire the defensive issues could be when the finals kick off in two weeks time.

"We've got some stuff to fix up in a really short time frame.

"We're going to give it our best shot at fixing it. The difference between our best performance and our worst is way too far apart.

"You're not always going to be at your best but at the moment we've got way too many guys that are nowhere near their best, all on the same night. That needs to change quickly."

Match Highlights: Roosters v Knights