A same-sex penguin couple is celebrating the birth of a baby chick from an egg they had been incubating and protecting in their nest for nearly five weeks.

The 91-gram chick was born last week at the Sea Life Sydney Aquarium to foster parents and gentoo penguins, Magic and Sphen, in what the aquarium said was an Australian first.

Tish Hannan, the supervisor of the aquarium's Penguin Department, was delighted with the way the pair had been looking after the chick.

"The first day, the younger of the two males was an absolute star, he fed the chick a lot," she said.

"The older male looked a bit disinterested at first, but as soon as he got on the chick he did an amazing job as well, so we are really, really proud of them both."

The egg was one of two from the biological parents and was given to the same-sex penguins after the aquarium observed they had developed a bond, calling and bowing to each other.

The had also built a nest out of pebbles.

Magic and Sphen showed strong bonding signs after being handed a foster egg. ( Supplied )

Ms Hannan said normally penguins would only raise one chick in the wild.

"A lot of the time the gentoo penguins don't have enough resources to raise both of those chicks," she said.

It was also not surprising Magic and Sphen were proving to be good parents.

"Both males and females have a strong urge to be parents and they share that parental responsibility 100 per cent between the two — it doesn't matter if they have a male-male or female-female pair," Ms Hannan said.

Same-sex relationships between penguins is not unusual, but a male-male pair incubating a chick in the wild is rare.

"With female-female pairs, what will happen is those females will actually go off and mate with males and come back and lay eggs and both females will raise the chicks.

"With the two males, sometimes they'll try and steal eggs that are unoccupied, so it can happen but it's not very likely," Ms Hannan said.

The chick stayed close to one of its dads. ( Supplied: Sea Life Sydney Aquarium )

It is too early to know if the chick born last week is a boy or a girl as there are no distinguishing external features.

A blood test will determine the sex when the chick is a little older.

Will the boys be given a chance to foster another egg?

"Not this year, one is enough parenting responsibility for them but it's very likely because they are doing such a great job that they're likely to return to each other next year and if they do that we'll definitely look at fostering them another egg," Ms Hannan said.