The 26-year-old, who played union as a child, was granted early release from his contract by the Parramatta Eels last November with a view to taking up a contract with the rebel R360 rugby union competition.
When the launch of R360 was put on hold, he tried to sign with another National Rugby League club, the Melbourne Storm, but the move was blocked in court by the Eels earlier this month.
A tall, athletic winger or centre, Lomax is exceptionally good in the air and a top class goalkicker - an area of the game where the Wallabies have struggled in recent years.
"This is a huge opportunity and a challenge I'm really looking forward to embracing," Lomax said in an RA news release.
"Stepping into a new code will push me as a player and that's something that genuinely motivates me.
"Like any player, the dream of one day representing the Wallabies on a truly international stage and potentially competing at a Rugby World Cup is a powerful motivation."
Australia host the World Cup next year.
Joseph Suaalii, who made the switch from league in a big money deal in late 2024, said last week that he thought Lomax would thrive in the 15-man game.
"He's a special player, you've seen what he's done in league," Suaalii told reporters.
"Zac has the aerial ability, he can kick a ball, he's strong, he's fast. He's a very good rugby league player. I'm sure ... he'd be a very good rugby player as well."
Suaalii's former team mate at the Sydney Roosters Angus Crichton will also switch to rugby union next year in a bid to make the Wallabies team for the World Cup on home soil.
A third Rooster, dual code test winger Mark Nawaqanitawase, has signed up to play club rugby in Japan from later this year, which would make him eligible for a return to the Wallabies.