A fatal shark attack at a popular northern Sydney beach has reignited debate over shark nets.

Despite being installed along the Northern Beaches just weeks ago, experts say nets do not always stop the ocean's biggest predators.

Mercury Psillakis, who was killed by a shark at Long Reef Beach on Saturday, was in the water where a shark net had been installed at nearby Dee Why as part of a rollout that included 50 other beaches from Newcastle to Wollongong.

What do shark nets actually do？

Shark nets are typically a 150-metre-long netting system, which sits at the top of water but are open at the bottom and to the sides.

Christopher Pepin-Neff works at Sydney University and has researched shark nets for almost 20 years.

"They're these gill nets that are set in front of the swimming area

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