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Welcome to the Bunnings Workshop community @saleens281. It's terrific to have you join us, and many thanks for your question about preventing water seepage.
Where were you intending to place this sheeting, on the inside or outside? I can't imagine sheeting will prevent this issue from occurring. Perhaps posting some images of the area so our helpful members can see what you're working with will assist. Let me know if you need a hand uploading some photos.
Water penetration is best resolved by waterproofing the exterior side of the wall, preventing the water from entering the wall in the first place. I understand access might not be possible due to the slope. If it is, drainage should be installed to take the water away and stop it from building up against the rear of the wall. You can also waterproof the back of the wall as a secondary measure.
What material is the wall constructed from? If it is masonry, you could remove any coatings on the wall and apply Dunlop 4L Damp-Proof Waterproofing. It resists hydrostatic pressure, which is water in the wall trying to push it off. Waterproofing the inside of the wall is a last-ditch effort as it traps the water within the wall and doesn't resolve the issue. The water will then travel along the wall and try to find another escape route.
Before applying the waterproofer, it would be an idea to seal any gap between the floor and wall with Sikaflex 11FC.
Please let me know if you have any questions.
Mitchell
