Emergency surgical treatment, which involves drilling through the skull to drain the blood and relieve the pressure, is life-saving and can prevent permanent damage.
In some cases a section of skull is temporarily removed to create a space into which the inflamed brain can swell without raising the intracranial pressure. The section of skull is later replaced.
A spokesman for St Vincent's Hospital in Darlinghurst, where Hughes was treated, said the surgery lasted less than an hour and the cricketer had been given drugs to place him in a temporary coma.
This can be vital if ruptured blood vessels are unable to deliver the usual amounts of oxygen and nutrients, as a comatose brain needs less oxygen to function.
Hughes is also likely to have been given diuretics to reduce the fluid in his body and anti-seizure drugs to prevent him suffering a fit which could cause additional damage.
The spokesman said it would be 24 to 48 hours before the outcome of surgery was known.