Oliver feared at first he had ruptured his Achilles tendon but an initial diagnosis revealed a strained calf muscle.
Highlanders doctor Stu McMain said on Thursday the results of scans confirmed the calf strain and that Oliver would be out for six weeks.
"The results didn't contain any surprises for us," McMain said. "So Anton will be out for six weeks as he goes through the rehabilitative period."
Oliver's lay-off will see the 43-Test hooker miss the All Blacks trial on June 3 and the Test match against Fiji on June 10.
The former New Zealand captain had been widely tipped to retain his Test jersey after impressing on the tour to Europe at the end of last year.
Keven Mealamu, Corey Flynn and Andrew Hore are all in the running to claim the hooker's spot for the first Test against the Lions in Christchurch on June 25 if Oliver fails to regain his fitness in time.