They aren't being cancelled as such, if you read the article, properly, they are reducing flights from Perth & Brisbane for a full 1 month only and 2 months only for Adelaide, before it returns to the same schedule as now.
Airlines do this all the time, because of load factors changing throughout the year.
Virgin is reducing flights to Doha out of Perth and Brisbane, cutting 2 flights from each city per week between Feb 16 and March 16. Reducing 7788 seats to 6362. Qatar meanwhile has reduced flights by 2 per week from Adelaide from April 13 and June 15.
United is about to do the same, lifting it's flights in October out of Melbourne to 5 days a week, then to 7 in December and then back down to 5 a week after the Australian summer period.