Initial reports from emergency services were that up to 100 people could be inside.
The call was just one of several fielded by the state's emergency crews this morning, after more than a month's worth of rain fell in some parts of Sydney.
There were reports of up to 48mm in some areas. Nine News reported that a number of weather stations in the Sydney Basin recorded their heaviest rain in six months within an hour, including Observatory Hill (41mm) and Canterbury (47mm).
The deluge caused widespread flash flooding, road closures and transport chaos.
NSW SES spokeswoman Becky Golling told news.com.au a member of the public phoned emergency services to report the precarious situation of the Marrickville apartment building.
"Police and also Fire and Rescue are on scene, assessing situation to see how serious it is," she said.
"They are trying to determine whether this building will collapse in to a water hole behind building."
Marrickville resident Geoff, who reported the incident to the SES, told Fairfax Media: "I've never seen anything like the rain today".
SES crews were called throughout the morning to rescue motorists from flood waters and repair damaged homes.
There were 13 calls for rescue, mainly in the city's inner west, which copped the worst of the rainfall and a further 140 to damaged homes.
Among them was a house in Tennyson Point, in Sydney's north west, where the front part of a home collapsed.
No one was home at the time.
