A further 27 detainees are still refusing food and water, officials said on Monday.
Four of the hunger strikers have sewn their mouths shut.
"As of this morning 18 people are in hospital being rehydrated and offered food," said a spokeswoman for Australia's immigration department.
"The number of people refusing food is 45. All are adult males."
The hunger strikers, Afghans and Pakistanis, are protesting against the rejection of their refugee claims by Australia.
More than 280 asylum seekers, including 93 children, are being detained on Nauru as part of the Australian government's
"Pacific Solution" to deter the arrival of boat people.
Australia's conservative government set up a fenced camp on tropical Nauru - a remote, 21sq km island - in 2001 when it toughened its policy against asylum seekers arriving by boat.
Human rights group Amnesty International has called for the government to close down the camp because it claims detainees are denied basic human rights and living standards.
Australia has one of the world's strictest immigration policies, detaining all asylum seekers, illegal workers and anyone overstaying their visas in guarded camps while their cases are handled - a process which can take years.
The hunger strike is the latest in a string of protests, riots, escapes and suicide attempts at Australia's five onshore and two offshore immigration centres, which are currently housing about 1200 people.