The 40-year-old Mehrtens will work on a part-time basis with the team's kickers Kurtley Beale, Bernard Foley and Brendan McKibbin.
An exciting running fly-half, Mehrtens played 70 Tests for the All Blacks and ended his career with 967 points, the most by a New Zealand player until Daniel Carter surpassed it on his way to becoming the highest points scorer in international rugby.
"Everyone's different in goal-kicking, it's about finding out what works best for the individual, whether it's to do with how you train, or when you train, or what you're doing technically in that or just your attitude, your mentality and your attitude towards kicking," Mehrtens said.
"At the end of the day, there are some pressure moments but goal-kicking, like anything in sport, should be about going hard and enjoying it.
"If I can do anything to help these guys achieve that, then great."
Mehrtens, who won five Super Rugby titles with the Canterbury Crusaders, will link with former Canterbury and All Blacks team-mate Daryl Gibson, who is the backs coach, at the Waratahs.
"I enjoyed my time playing alongside Daryl Gibson and I've known (head coach) Michael Cheika for a long time since I played against him in Italy," Mehrtens added.
"I like both those guys, I think they're awesome coaches and I like what they seem to be trying to doing here as well.
"It's just a pleasure to be involved and not working full time in rugby now, I've actually got a passion for it."
The Waratahs open their Super Rugby season against fellow Australian side the Western Force in Sydney on February 23.
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- All Blacks
- Bernard Foley