New Zealand under-20s rugby coach Rob Penney predicts a hugely physical final against South Africa as the Baby Blacks look to win a fifth consecutive world title this weekend.

New Zealand beat Wales 30-6 in their semifinal in Cape Town  this morning, gaining revenge over the Welsh who had earlier won their pool encounter.

South Africa won the other semifinal 35-3 against Argentina to set up a dream final to be played on Saturday morning (4.30am NZ time).

"South Africa will be a physical side with an obvious home ground advantage," Penney said of the final that could attract around 40,000 fans to the famous Newlands ground.

"The importance of this match is not lost on us.  We know it will be a huge challenge to play against the crowd favourite, and we expect that crowd will be loud.

"The focus for us now is on maintaining discipline and to have a solid week preparing for the final."

Penney believed his team could take a lot of confidence out of their semifinal win where New Zealand scored crucial tries either side of the interval and then dominated the second half.

"I don't think we are a side that is overly confident in what we are doing and that performance today, I think, will certainly give us a little bit of belief which is important," he said.

"We're really pleased with the effort the boys have put into preparation over the last few days and today we saw the benefit of a real shift in momentum and attitude from the team.

"I was just really rapt that the boys had been able to grow over the last 10 days or so. I think tactically they did a really good job and got the right outcome so it was great.

"Certainly that was a massive bonus for us getting those tries either side of halftime, it really ripped the heart out of the Welsh."

New Zealand co-captain Bryn Hall was delighted with the way his team hit back from the disappointment of losing their pool match to Wales. That had provided extra motivation for this play-off game.

"For sure, it was very, very disappointing last time we played Wales though I guess we have learnt our lesson," Hall said.

"We executed our game plan very well today, we played with a bit of width and we played much better with our kicking. The last time we played unfortunately we didn't use that facet of play so we played much better today.

"We always knew that Wales were going to come and fight, they are very, very passionate - just like New Zealand, very passionate on their rugby - so we knew that first 20 minutes were very important, trying to weather that storm.

"Those two tries just before and just after half time were very crucial for us. We talked about it a lot through the week saying that last 10 minutes and first 10 minutes after half time were very important."

South Africa coach Dawie Theron suggested the final would be a huge occasion.

"Well it's going to be tough. Both are rugby nations and both of them want to get their hands on the trophy and that's why it's going to be a real tough occasion. It's going to be a wonderful but tough rugby match," Theron said.

South Africa captain Wian Liebenberg said the Baby Boks would have to get stuck into their homework on the New Zealand team.

"Well obviously we have to go into the week and look at some of their games and do our  preparation," he said.

"We expect a physical battle from them, it's always going to be physical. I mean everyone loves New Zealand and South Africa playing against each other so it's going to be a good game, I'm really looking forward to it."

The final will be broadcast live on Maori Television with coverage from 4:30am, and replayed at 7am.

- © Fairfax NZ News