Image: Caili Christian / Twitter

A man is in custody after allegedly flooding 18 levels of a Southbank apartment building.

The man allegedly turned on all fire hydrants from levels 1 to 18 of the high-rise on City Road.

It causes the building to flood and prompted 120 residents to evacuate about 9.30pm.

Something big is going down. #Melbourne #southbank pic.twitter.com/GQbJxfMA2k

— Caili Christian (@cailichris) May 27, 2018

MFB crews were able to turn off the hydrants before police found the alleged culprit, a 27-year-old man, hiding inside a room.

The scene was deemed safe about 10.20am.

However a man, 47, was injured while working on one of the damaged hydrants about 4.30am today.