The best deals are always well in advance....That afternoon flight to Karrath must be a special as all Sundays for months have the cheap seats sold only leaving $789 ones.
Flight before is $403
If you look at Bris to Townsville for tomorrow or Sunday its $321 or $368 depending on which arvo flight you take,but if you book well in advance can get for $217.
As for Karratha and the rest of the Pilbara,certain days and certain flights all year you cant get the lowest fare.