The unnamed baby penguin, known for now as Baby Sphengic, was born on Friday afternoon, weighing just 91g.
It will live in a nest built by the two gentoo penguins, Sphen and Magic, who bonded ahead of the 2018 breeding season.
At the aquarium’s exhibit, gentoo penguins keep their eggs warm on pebble nesting rings.
Staff noticed the pair were collecting ice pebbles for a nest of their own and gave them a dummy egg.
The penguins showed strong nurturing skills and were soon entrusted with a real one.
Staff selected the egg from another couple who had laid two during the breeding season.
“The loving foster parents are co-parenting exceptionally well,” said a statement issued by the aquarium after the chick was born.
“Baby Sphengic has already stolen our hearts!” Tish Hannan, penguin department supervisor at Sea Life Sydney Aquarium, said.
“We love watching the proud parents doting and taking turns caring for their baby chick.”
“With that said, the first 20 days of a penguin chick’s life are the most vulnerable, so it is extra important the chick is very happy, healthy and well fed by his parents.”
The chick will stay with its adopted parents for the first five to six weeks of life, with the penguins feeding it up to 10 times a day.
Once it is big enough, it will start to lose its baby fluff and start growing adult feathers.
At that point, the chick will learn to swim.
Ms Hannan had previously said gentoo penguins share parenting and feeding responsibilities equally, so there is little difference between opposite and same-sex parenting.
Same-sex relationships between penguins have been observed at other zoos.
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