former union official was arrested after returning from the middle east .
he left the country in january to illegally join the fight against is .
it is understood matthew gardiner headed to iraq or syria to join kurdish .
the australian federal police has confirmed it is investigating the case .
the 43-year-old was allowed to leave the country because he was not on any watch list , report says .
a spokesperson for attorney general george brandis said : ` if you fight illegally in overseas conflicts , you face up to life in prison '