Mains, the last coach to take New Zealand to a World Cup final in 1995, said the All Blacks did not extend themselves during pool games.
"We saw 20 minutes of very good rugby against France," Mains said.
"But they stepped it up against Australia with a lot of commitment. There was no way they were going to let that game go."
He was impressed by the tactics used in the semifinal.
"They exposed and exploited the weakness in the Australian team in the tight five and Quade Cooper. "
Mains is confident the All Blacks will beat France because there are enough players who have been through it before.
"They will have slightly different tactics," he said. "But they shouldn't change the way they prepare."
Mains does not think the French team is good enough to give the All Blacks a serious challenge.
"They don't have the world-class attacking players we have seen in the past. If the All Black team is on song I don't see how France can come within 20 points of them. They are not the second-best team in this competition."
Mains said coach Graham Henry was not going to repeat the mistakes made in 2007. That was vindication of the reappointment of the same coaching panel.
He rated blindside flanker Jerome Kaino as the best and most consistent All Black at the World Cup.
"It's hard to pick out other individuals because it's been a darn good team effort."
The other teams to impress Mains were Wales and South Africa, teams he thought were taken out of the tournament by bad refereeing decisions.
"I would have felt a lot better about winning the World Cup if Wales or South Africa had been the other team in the final," Mains said.
"They would have been worthy opponents for the All Blacks."