Western Australia police commissioner Karl O'Callaghan said a "large number" of the 200 on board were feared to have perished.
"Some of the very early reports suggest that up to 75 people may have drowned, but I do stress that they are unconfirmed at this stage," O'Callaghan told reporters.
"There were about 200 refugees on board we think. Currently there's about 40 on the hull and the rest are in the water," he added.
O'Callaghan said police were "very concerned for a large number of people who may have drowned" as a result of the capsize, which occurred in Indonesian waters 120 nautical miles north of Australia's Christmas Island.
"We know from what we've been hearing from the aircraft that there's not 200 life jackets on board," he added.
O'Callaghan said it looked as though the operation "will have to go well into the night".
Approximately 110 persons from a capsized boat rescued by Indonesian authorities.
According to the Indonesian officials the identities of those rescued could not be ascertained yet due to this it's unable to release informations about SriLankans.