An investigation has been launched after a grandfather mistakenly took home the wrong child from a daycare centre in Sydney, Australia.

The man arrived to collect his grandchild from First Steps Learning Academy in the southern suburb of Bangor, on Monday afternoon.

But he accidentally took home a different child, who was asleep in a dark room.

The mistake only became clear when the mother of the child arrived at the centre to find that her one-year-old was not there, the Sydney Morning Herald reported.

"I can't explain the feeling," she told the newspaper.