Rachin Ravindra and Kane Williamson ground out unbeaten centuries as New Zealand punished a second-string South Africa on the opening day of the first Test at Mount Maunganui on Sunday.

A Proteas side featuring six debutants paid heavily for deciding to bowl and spilling chances as the home side reached 258-2 at stumps.

Williamson was at his gritty best in reaching 112 off 259 balls, his 30th Test century, while Ravindra reached three figures for the first time and was unbeaten on 118 from 211 deliveries.

The pair combined for a stand of 219, wearing down an attack which began the day brightly after captain Neil Brand, on debut, opted to field on a green-tinged Bay Oval pitch.

Seamer Tshepo Moreki trapped Devon Conway leg before wicket with his first ball for South Africa before Dane Paterson removed Tom Latham to leave New Zealand 39-2.

Williamson and Ravindra survived some anxious early moments before settling and their patient approach was rewarded on a pitch that flattened out.

There were just two loose shots and the pair will resume on Monday needing 38 more to post the highest partnership for any New Zealand wicket against South Africa.

Williamson was on 45 when his skied shot off Ruan de Swardt was spilled by Edward Moore running backwards at extra cover.

Seamer De Swardt was unlucky again when Ravindra was dropped by Duanne Olivier at deep square leg on 80.

Williamson went on to post his fifth century in his last six Tests, lifting his career average above 55 to cement his status as the ICC's top-rated Test batsman.

Left-hander Ravindra's knock came a week after he was named the ICC men's emerging cricketer of the year, following his prolific run-scoring at the 50-over World Cup in India.

The six Proteas debutants are Brand, fellow-opening batter Moore, Raynard van Tonder, de Swardt, wicketkeeper Clyde Fortuin and Moreki.

While the newcomers all boast considerable first-class experience, the team is raw at Test level, with pace bowler Olivier's 15 caps making him the most experienced player.

Nearly all of South Africa's first-choice players were unavailable to tour because of their contractual obligation to play in the country's domestic Twenty20 league.