The 26-year-old actor, who sought treatment in rehab for cocaine addiction twice last year, recently began dating his 'Bad Neighbours' co-star after meeting her on the set of the upcoming comedy.
A source told E! News: "They are definitely an item. They were seeing each other a bit during filming of their movie together and ever since he went to rehab they got a lot closer.
"Her friends love him and say he is so sweet. He's really good to her."
The heartthrob and the 20-year-old former Nickelodeon star were photographed laughing and talking animatedly as they watched a Los Angeles Lakers basketball game together earlier this month.
News of their romance comes just weeks after Zac sparked concern about his sobriety after he and his bodyguard were involved in a fight with a group of transients in downtown LA.
But the 'That Awkward Moment' star, who previously dated Vanessa Hudgens and Lily Collins, has reportedly hired a health and wellness coach to help him stay on track.
A source said: "Zac's committed to his sobriety. It's an ongoing process."