Under-pressure New Zealand captain Kane Williamson received a huge vote of support from an unlikely source today.

South African skipper Faf du Plessis, speaking out for the cricket captains' union, defended Williamson, who had his worst test as a batsman - two and one -- in the eight-wicket loss at the Basin Reserve last week and with hefty criticism of his team's performance on what proved the third and final day.

Du Plessis knows the score and had no problem singing Williamson's praises today.

''I think it's unfair. I think he's done really well with the New Zealand team," du Plessis said.

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''He's a fantastic player. I've read a lot of times that he's the best No 3 in the world.

''It comes with not winning. It's part of our job as captain and sometimes it's not nice, sometimes you feel it's not deserved.

''The New Zealand team have done some really good things in the last while. The one-dayers were a huge win for us especially with their track record and for the tests it's really important we win the series."

Du Plessis admitted the loss of pace bowling spearheads Tim Southee and Trent Boult to injury was a massive blow for New Zealand but he would expect the replacement players to come in and perform strongly.

He is sure the Seddon Park pitch will turn during the tests.

In both ODIs on the ground earlier in the tour there was help for the spinners. Du Plessis expected more in the coming days.

''It looks a little bit different (from those two pitches) slightly greener grass covering.

''Even with that, you might get sticky turn, where the ball grips into the moisture and as the game goes on we know it will change colour, go back to a brown tinge, then it will spin."

No decision has been made on whether injured wicketkeeper Quinton de Kock will play, after damaging his right index finger in the second test at Wellington.

Du Plessis however hinted that as it's an important match he may be persuaded to play and put up with the pain for one last match before he takes time off.