Sydney - An Australian nurse alleged to have given medical assistance to Islamic State militants fighting in Syria could face arrest for violating tough new security laws when he lands in Sydney on Friday, police and officials said.Australia is on high alert for attacks by radicalised Muslims, including home-grown militants returning from fighting in the Middle East, having raised its threat level to "high" and unleashed a series of high-profile raids in cities.An Australian national, identified by Foreign Minister Julie Bishop as Melbourne-born nurse Adam Brookman, is returning home voluntarily after negotiating with the government, the Australian Federal Police (AFP) said.Brookman has not been charged with any crime, police said, but is subject to investigations that could see him face up to a decade in prison if convicted in a crackdown on radicalism launched by Prime Minister Tony Abbott."If there is evidence an Australian has committed a criminal offence under Australia law while involved in the conflict in Syria and Iraq, they will be charged and put before the courts," an AFP spokesperson said in a statement.Domestic spying capabilitiesSecurity analysts have put the number of foreign fighters in Iraq and Syria, travelling from scores of countries around the world, in the thousands.Abbott has told parliament at least 70 Australians were fighting in Iraq and Syria, backed by about 100 Australia-based "facilitators".Abbott has secured a raft of new security powers in recent years, expanding domestic spying capabilities and proposing to strip citizenship from dual-nationals accused of committing violent militant acts.Brookman, a father of five, told Australia's Fairfax Media that he travelled to Syria on a humanitarian mission and was forced to join the militant group after being injured. He said he fled into Turkey after witnessing atrocities.Bishop said Brookman may have violated laws passed last year that mean Australian citizens now face up to a decade in prison for travel to overseas areas declared off-limits."Any Australian who supports or fights with ... Islamic State is potentially committing a crime against Australian law, including our sanctions regime," she told reporters."So Mr Brookman is obviously a person of interest to our law enforcement and intelligence agencies."Abbott this year ruled out an amnesty for Australian citizens seeking to quit foreign militant groups and return home, in the wake of media reports that his government was negotiating with potential defectors.