AUBURN HILLS – Prevent the surf from reaching the shore with a sponge. Stop the 8:15 out of Grand Central with a piece of twine. And then try the really tough task: keep Detroit’s Ben Wallace off the offensive glass.

That was center Jason Collins’ task as the Nets started the Eastern Conference semis against the Pistons here last night.

“Definitely, I’ve got to keep him off the offensive glass,” said Collins, who was paired against the NBA’s second-best offensive rebounder and third-best overall boarder.

“He’s one of the best rebounders in the league. I played against him last year and I did a good job. I’ve just got to do another good job of just keeping him off the offensive glass.”

If not, here’s your sponge and twine.

Wallace, at 6-foot-9, 240 pounds, is simply a fearsome specimen. Collins faced him in three regular-season games (he missed the fourth with a bum foot) and limited Big Ben to 12.3 points and 12.3 rebounds.

In last year’s playoffs, though, Wallace gobbled up the Nets on the glass for 17.3 rebounds a game.

“It’s positioning, his athleticism. It’s a lot of things,” explained Collins, who won’t be asked to produce numbers as much as blockouts for others to get numbers.

“It’s a team effort. But I know I have to definitely keep it in mind to find him as early as possible and keep him away from the basket.”

And the Nets figure to look to Collins for some of his little face-up jumpers in an attempt to lure Wallace away from the lane. Not a bad idea: Wallace also ranked second in the league (to Portland’s Theo Ratliff) in blocked shots with 3.04 a game.

“He is a shot-blocker – when you are a shot-blocker, you take calculated risks,” said Net coach Lawrence Frank. “And if you look at the amount of times he goes for it, his percentage is pretty good.”

And as a rebounder?

“Ferocious,” Frank said.

Said Collins: “He’s just active. He’s got long arms and he does a good job fighting for the ball.”