E-book, probably read in the last year or two. Probably written in the last 10 years. Seemed to be first in a series.
A comet or small asteroid is reported to be making a near miss to Earth. But as the time draws near, its changed to reporting it was actually going to hit all along.
Story is told from the perspective of a teenager (last year of school, or just graduated) who's parents were away on an isolated retreat and don't hear the updated reporting.
His town survives the near misses (pretty sure there were multiple impacts as it broke up. (Europe, Middle East, India, Indian Ocean and Australia) He lived in Western Australia, and the tsunamis from the ocean impacts destroy most of western Australia coastal cities/towns. Half of his town gets flooded.
Believe the story was written as journal entries, as he describes trying to survive in the heavy ash fallout from the impacts, and scrounging for supplies to protect himself and his house.
Eventually he gets a housemate, (the popular girl in school who never really noticed him) and builds a homemade hover craft to escape the town. They plan to head east to get away from the ash, and other residents who are becoming desperate and dangerous as supplies start running low. The book covers about a 1 year period, and he is stocking his hovercraft as the book ends. (Cant remember if left town before the book ends, but I think he does)