Australia\'s troubled Christmas Island detention centre was rocked by a second night of unrest, officials said Thursday, with police firing tear gas and non-lethal rounds at rioting inmates. Immigration Minister Chris Bowen said between 20 and 40 immigration detainees \"engaged again in vandalism and violence\" on Wednesday night, with a building set alight at the remote Indian Ocean centre. \"The federal police again took action and they used, I\'m advised, gas and bean bag rounds,\" Bowen told reporters. Tear gas, flash and acoustic weapons and \"bean bag\" or fabric rounds filled with lead shot were fired on about 50 detainees during riots Tuesday night, the second time in as many months Christmas Island has erupted in protest. Australia has a mandatory detention policy for asylum seekers arriving by boat, most on leaky vessels through a popular people smuggling corridor from Indonesia. Christmas Island is the main facility, and home to 600 inmates. A record influx of boatpeople -- almost 7,000 in 2010 -- has stretched facilities to capacity, prompting the government to release women and children into the community and open new centres on Australia\'s mainland. The large numbers have resulted in lengthy processing delays, leaving some detainees locked up for 18 months or more and tensions running high. Canberra strengthened the character test for asylum visas so anyone convicted of crimes, including rioting, in detention could be shipped back home following wild riots on Christmas Island and in Sydney earlier this year. Bowen warned detainees they faced the \"full force of the law\" if the violence continued. \"This sort of activity is completely inappropriate, it is way out of line with the expectations of the Australian people,\" the minister said. \"Undertaking this sort of activity is achieving nothing except potentially their release from detention and transfer into a prison.\" At least 30 detainees have already been charged over riots on Christmas Island last month and in March and violent protests at Sydney\'s Villawood in April in which nine buildings were torched. The latest unrest came as human rights advocates warned that the mandatory and prolonged detention of asylum seekers was a serious breach of Australia\'s international human rights obligations. Australia\'s Human Rights Commission said the mandatory detention system was fundamentally flawed because it set no limit on how long people could be locked up and the detention can not be challenged in court. \"People in detention often express disbelief and a sense of injustice that in a country like Australia, they could be detained indefinitely without the ability to challenge their detention before a judge,\" said Commissioner Catherine Branson. \"We know from bitter experience that prolonged detention causes serious mental harm.\" Branson said there were about 4,000 people currently in immigration detention in Australia, including children.