Waratahs star Rocky Elsom is wary of the Stormers array of skilful ball runners in what he believes is a potential semi-final preview in this weekend's crucial Super 14 rugby clash in Cape Town.

Wallabies flanker Elsom lavished praise on the red-hot Stormers, who head into Saturday night's clash having won seven of their last eight games.

The second-placed Waratahs head into the penultimate round clash at Newlands just two points and as many positions above the Stormers.

A victory would keep NSW on track for a home semi-final, while a loss could force them out of the top four.

If the Stormers win, they would move up to either second or third with just a final round away game left against the last-placed Lions, while the Waratahs will have to beat old foe Queensland in Brisbane to have any chance of making the semi-finals.

"It's a vital game (against the Stormers) because we need to win given that we are all so close on the ladder," Elsom told AAP from Cape Town.

"We've a chance of playing them again (in the finals) if we loose and even if we win. There is a lot of reasons why we need to win."

NSW suffered a blow with confirmation Wallabies prop Matt Dunning would miss Saturday's game because of a calf strain he suffered in last weekend's loss to the Bulls.

"I'm hoping to be available for the following week against the Reds," Dunning said.

"It's really disappointing to miss this match. It's such a big game, both teams have been going well and there's a lot at stake."

After fielding five forwards, including two props on the bench last week, NSW coach Ewen McKenzie is expected to revert to a more traditional 4-3 split this week.

That could allow back Timana Tahu to make his long awaited return from injury.

Elsom expected the Stormers to pose different problems to the Bulls.

"I think the Stormers have a lot better footballers in your classic sense where they create a lot of options and they play a little wider of the ruck and individually are a little bit harder to handle because they are strong ball runners with good skills," Elsom said.

"I think that's probably why they are going so well.

"They have got a lot of players who can break tackles and break the line and they are capitalising on that.

"Guys like Schalk Brits and Jean de Villiers are really making inroads into the opposition and have been making themselves a handful to defend.

"All of their players seem to be in sync with each other and just have a really attacking mindset and I think that's probably why they are looking so strong."

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