Beauden Barrett's performance after replacing incumbent Dan Carter in the All Blacks' victory over the Springboks at Eden Park has highlighted the extent of the production line for the No10 jersey.
Barrett, Aaron Cruden, Tom Taylor and Colin Slade are all waiting in the wings, with Mains saying it would give the team a boost ahead of their 2015 World Cup defence.
"It's a great time for New Zealand rugby right now, especially as we're halfway to the next World Cup, having this many resources to choose from," Mains said.
"We've just got such a wealth of riches at the moment at No10, when you've got five first-fives floating around you're really lucky."
Mains said he had never seen depth like it.
"Dan Carter is quality and obviously first choice when he's available. But there's a bit of a scrap with the guys behind him, and that's how it should be. That's healthy and keeps everybody on their toes.
"When you've got someone with the quality of Colin Slade near the end of the line, who's complaining?"
Rugby analyst and former All Blacks selector Peter Thorburn had no doubts over who he would have playing at No10 should Carter be taken out of the equation.
"Aaron Cruden has fantastic vision on attack, and his ability to find space in behind the ruck with clever kicks has impressed me.
"Cruden's defence is top class as well, he can tackle and read the attacking line just as well as Carter.
"That's not to take anything away from Barrett, he's just not had the opportunities at international level because Cruden has been so reliable."
Thorburn said the young talent coming through in the ITM Cup was giving the All Blacks a huge boost compared with Australia, who have no domestic equivalent.
