Fast bowler Tim Southee has been ruled out of New Zealand's two tests against South Africa due to a thumb injury.

In a major blow to New Zealand's chances of being competitive against the world No 1 test side, Southee will be sidelined for 6-8 weeks after rupturing a ligament in his left thumb during the Plunket Shield match between Northern Districts and Wellington at Karori Park on Wednesday.

He suffered the injury while diving in the field and scans revealed it was more serious than first thought.

"He landed heavily on his thumb while fielding, with scans indicating the ligament has fully ruptured and retracted,'' New Zealand team physiotherapist Paul Close said.

"After consultation with NZC medical staff and a hand specialist it was decided the best course of action is for him to have surgery. He will commence rehabilitation immediately following the procedure, with a view to him returning to cricket during the home summer. The procedure is likely to keep him out of the game for 6-8 weeks.''

A decision on Southee's replacement is yet to be made.

Southee is due to become a father today or tomorrow and was already missing the three-match T20 series. But he was a leading contender to be new captain Brendon McCullum's deputy in next month's two tests.

It's a further setback for New Zealand as they face South Africa without former skipper Ross Taylor who is sitting out the tour.

Southee has been outstanding in New Zealand's recent tests, taking 8-120 in their win over Sri Lanka in Colombo last month.

- © Fairfax NZ News