"But if he was sentenced to 18 months in July, I'm not sure why is back out again after only 6 months ? "
Quite simple, Mikey.
Firstly all prisoners serving determinate sentences are released at the halfway point. This is automatic and unconditional. So this knocks his sentence down to nine months. Secondly, almost all prisoners sentenced to four years or less are entitled to release under the "Home Detention Curfew" arrangements. Those sentenced to twelve months or more are eligible for release after serving 90 days less than half their sentence. So, ninety days off the nine months is six months. That's why time flies !!!