Australia's Sydney Aquarium celebrated its 20th birthday this month with special guest, Olympic swimmer Brooke Hanson, cutting a shark-shaped cake and receiving a kiss from Burleigh the seal.

Since opening in 1988, the aquarium has welcomed more than 21 million visitors and doubled in size, including the creation of the Great Barrier Reef exhibit, a revamp of Australia's only all-natural Seal Sanctuary and the introduction of the glass-bottomed boat ride, 'Shark Explorer'.

Sydney Aquarium CEO Kevin Bush said reaching the 20-year milestone was a testament to the attraction's ongoing appeal, with one in three international tourists to Sydney visiting the aquarium.

"In 1988 we opened Sydney Aquarium with 100 vertebrate species. We now have more than 350 and our curatorial staff numbers have doubled from eight to 16 over the same period as we continue to offer visitors new things to see and do," he said.

To celebrate its birthday, free Nemo, seal and shark face masks are available for kids throughout the July school holidays. Special family discounts are also available for entry to Sydney Aquarium and Sydney Wildlife World.