Can it be that “Australia” is only Baz Luhrmann’s fourth film? His uniquely Australian style, which mixes broad comedy and campy performances with sweeping romance and beautiful photography, is so clearly defined that it feels like it must have taken more than just “Strictly Ballroom,” “Romeo + Juliet,” and “Moulin Rouge” to establish it.

But nope, “Australia” is only No. 4 for Baz, and it’s easily his most ambitious project to date, a 165-minute epic adventure that’s part Western, part war drama, all Aussie. It incorporates the country’s notorious treatment of its aboriginal peoples — until the 1970s, children, especially those of mixed race, were still being taken from their homes by the government — into a lively, rollicking, overlong, overstuffed movie that tries my patience as much as it entertains me.

It begins in 1939, which for much of Australia apparently looked and felt a lot like the American Old West of 60 years earlier: cowboys, saloons, dusty prairies, and a cattle-based economy. Like many American Westerns, “Australia” concerns a prim aristocrat lady teaming up with a rough-and-tumble cowpoke, whereupon hilarity and opposites-attract romance ensue. Luhrmann has not exactly reinvented the wheel here; he has merely re-spun it (counterclockwise, of course, owing to the Coriolis effect).

The fancy lady is Sarah Ashley (Nicole Kidman), a Brit whose husband owns a ranch called Faraway Downs in northern Australia. Compelled by the family’s crumbling finances to sell the place, Sarah heads south (dragging numerous pieces of luggage and hatboxes) and discovers that Faraway Downs has been the object of sabotage and treachery by the Carney Cattle Company, which has a near-monopoly on beef production and has sought to devalue the Ashley property in order to buy it cheap. Lady Sarah will not stand for this!

The task that must be accomplished in order to save Faraway Downs and the Ashley family is to drive 1,500 head of cattle all the way to the city of Darwin, where they can be sold to the army. To help, Sarah enlists a man whose name and occupation are both Drover (Hugh Jackman). He’s a freelance mercenary in the old style, beholden to none, reluctant to help anyone other than himself, and so forth. You know the type. He and Sarah are opposed not necessarily by Carney Cattle Company’s namesake, King Carney (Bryan Brown), but by his minion, Fletcher (David Wenham), who behaves nefariously and twirls his mustache, at least metaphorically.

The film is rife with stooge-like humor at first as the opposing characters are introduced, then eventually settles in to a more traditional Western sensibility. It remains distinctly Australian, however, with the inclusion of an aborigine boy, Nullah (Brandon Walters), a precocious lad who, like many of his generation, is the offspring of a black woman and a white, adulterous man who now lives it up in the city, his secret progeny safely hidden in the outback. Nullah and his mother have worked at Faraway Downs for who knows how long, and he’s eager to help “Mrs. Boss” and Drover fight back against the goliath Carney company.

As a 90-minute old-fashioned Western, “Australia” is just fine. Alas, it goes on for at least another hour. Yes, once the Western portion of the evening’s entertainment has concluded, “Australia” launches into its World War II segment, focused on the Japanese Navy’s attack on Darwin not long after Pearl Harbor. Thematically, the film needs this chunk to fill out its Nullah-related ideas; from an entertainment standpoint, it really would have been better if it had just been a story about Sarah and Drover movin’ cattle across the plains. The rest feels like an afterthought, an extension whose length is unjustified by its content.

Luhrmann has tried to tell too much of a story in one movie, and his enthusiasm far outpaces the audience’s. Many of the special effects look plastic, which might be Luhrmann’s style — “Moulin Rouge” looks artificial, too — but it’s more likely the result of budgetary concerns and time constraints. Luhrmann was still tinkering with the film days before its release, further evidence of his overreaching ambition.

“Australia” turns out to be a little messy and a lot too long, and Nicole Kidman doesn’t register much personality. Hugh Jackman is a lot more fun to watch, grinnin’ and ridin’ and horsin’ around; so is 12-year-old Brandon Walters, in a very natural, unforced performance as Nullah. In the end, the big ol’ goofy flick is more good than bad, a dazzlingly sunny attempt that may tide Baz fans over until project #5 comes along.

B- (2 hrs., 45 min.; PG-13, a little mild sexuality, mild profanity, one F-word, some war-related violence.)