The Refugee Action Coalition said about 75 asylum seekers striking on the outside oval have been joined by some 300 others inside the compound, with seven stitching their lips together.
"They said overwhelmingly what they're getting is simply more threats and more warnings about being sent offshore, being sent to Manus Island," Refugee Action Coalition spokesman Ian Rintoul told SBS.
SBS reporter Kristina Kukolja speaks with Ian Rintoul of the Refugee Action Coalition.
The protest began six days ago, reportedly in support of Iranian asylum seeker Reza Barati who died during riots on Manus Island on February 17.
Authorities have locked down compounds on Christmas Island in order to manage the hunger strike.
Mr Rintoul said people inside the compound are missing medication and have joined the strike by default.
"The fact that the department responded by locking down the compound has meant that hundreds of people who weren't directly involved in the protest have been locked inside their compound for several days and really the protest spread," said Mr Rintoul.
Immigration Minister Scott Morrison has confirmed the protest and says it is "under control."
"It is a fairly low level activity, it's not uncommon to have protest activity in detention centres," said Mr Morrison.
