The Cook Islands led 6-4 before Australia completed an unbeaten pool campaign with a 66-6 win in Townsville on Saturday.
Reece Walsh was preferred to James Tedesco at fullback, while Tom Dearden edged Sam Walker at halfback.
Walsh scored and supplied five try assists despite pain after a bump to his surgically repaired left ankle; Walters said he and late withdrawal Cameron Murray, who had a sore calf, were not in doubt next week.
"That's Reece, that's what he brings to the team," Walters said.
Walters kept Tedesco in contention, saying, "I'll make a decision about the fullback in the next couple of days."
Dearden provided two first-half try assists and combined well with Cameron Munster.
Jacob Preston made two line breaks and scored, while Jack Bostock and Tom Trbojevic each registered hat-tricks.
Australia's completion rate was below 70 per cent, a concern heading into the knockout stage.
The Kangaroos face either Samoa or Tonga in their semi-final in Sydney on November 8.