I have fond memories of the books, which I read several years ago, and it's difficult not to let a comparison of book vs. film colour the review. I'll get the one comparison I really would like to make to the books out of the way straight away. I felt like the movie failed to capture some crucial elements of the atmosphere of the books, and what made them good reads. The books were very heavy on realism. The movie turns up the action a little bit higher than the books, with a stronger focus on cool explosions and excitement than almost purely on the psychological horrors of war. This is still absolutely a factor in the film, and is handled relatively well, but I felt like the greater emphasis on action in the movie hurt the realistic feel that was present in the books. Similarly, a lot of detail was omitted that's present in the books that focuses on the more practical aspects of what's going on. These changes are understandable though, and serve to make the movie better paced and more marketable. Essentially what I'm saying is that, while it'd be easy to be critical of the differences in tone between book and film, the movie is probably better, and more appealing to a wider audience, thanks to these changes.
The film's script is one of its strong points. In such a serious, grim war-drama kind of film I wasn't expecting anywhere near as many laughs as I got out of it, and yet the film manages to get away with it without feeling like a parody. There is a particularly memorable scene towards the films finale which was simultaneously very fun and extremely tense - not an easy combination to pull off.
The actors are generally Good Enough. There are a few stand-out moments where the script is really brought to life and you become extremely immersed in their performances, but there are also moments where things feel a little forced or unnatural. I think going with (mostly) unknown and inexperienced actors was a good choice for the type of film Tomorrow is, but the flip side of that is that occasionally their lack of experience shows. For the most part, the performances were competent, leaning towards good.
The film's direction and production are generally fairly standard. I like the visual style employed by the film, and some of the characters and locations really meshed with the way I imagined them from the books. There are a few choice shots in here in terms of composition, and the action scenes are well shot in that they're easy to follow and exciting to watch.
I worry that this film will be misunderstood as something that aims to demonise Australia's international neighbours, and I'm sure it's a take that we'll see in the media, but I think such an interpretation is somewhat unfair, and there are a few scenes in the film that make it abundantly clear that this isn't the wish of the filmmakers. Nor was it ever the wish of the original author, which is why details about the invaders are sparse in the books. This aspect has been preserved in the film, however I think the issue of race will definitely be sensationalised to a degree here. In some ways it might be a worthwhile film to see because the fallout might be not insignificant and worth understanding.
Tomorrow When The War Began is all around a solid film with a very good script, decent action scenes, and decent actors, all of which comes together in quite a neat little package. I think this movie will definitely stand on its own even for people who haven't read the books. I recommend Tomorrow When The War Began, and I hope they do well enough to carry through with their plans for the rest of the series - with some maturation, this could become a pretty outstanding set of films.