Can someone help me.

My mother has 3 years visa, issued on 19th may,14 which has 12 months stay in 18 months. Condition on Visa Last day to arrive 19th may 17.
First visit to Australia 16th June14 to 8th Feb 15 = ( 8 Months in Aus)
Stayed overseas (10 months 8 days )
Second visit to Australia 16 dec 2015 to 17 July 16 = (7 months 1 day in Aus)
After reading here & and looking at visa, I was in impression of - if my mother stays overseas 6 months she can further stay 1 year.
Today migration agent said to me as per vevo my motherís visa is expiring on 19 may 1,7 so she must exit before 19th may 17.
Is there anyway i can be 100% sure can she stay for another 1 year or just 5 months after gap of 6 months. Any official email address where i can get reply related to this.
If she can only stay for Five months can she apply for another Visa from overseas, even when present Visa is active.