I've read lots of comments in the Forum to the effect that plastic spars in big models (like this one) ought to be replaced with wood or (in the case of those molded in halves) reinforced with metal tubing inside. I have to say that I've never found it necessary in big kits. But I hasten to offer one large caveat: I haven't built one of those kits in a long time. In recent years I've heard all sorts of horror stories - which I believe - about severely warped parts and low-quality, overly-flexible, or overly-brittle plastic. Neither of those was a major problem in the Goode Olde Dayes.
I think the idea of "pinning" the broken plastic part is worth trying; you certainly have nothing to lose. Get hold of a piece of brass or steel wire that's about half the diameter of the broken part. Pick a drill bit that's a little larger than the wire. Using either a slow-turning motor tool or a pin vise, carefully drill a hole in each fragment of the broken part. The deeper the hole the better; as a general rule, make it at least twice as deep (in each fragment) as the diameter of the spar. Cut a piece of the wire to the proper length, and put the assembly together with gap-filling CA adhesive. (The reason for making the holes slightly oversize is to allow for a little adjustment, in case the holes don't line up precisely with each other. The CA will fill any void.)
The worst-case scenario I can see here is the possibility that some of those modern, cheap plastics may not react well to CA and/or drilling. (I've heard stories of waxy surfaces, and even mica-like ones that disintegrate into thin layers.) If something like that happens, the only thing I can suggest is to try CA accelerator. That stuff cures lots of problems.
This trick usually works. Once the rigging is in place, it will hold the fragments of the spar together even if, for some reason or other, the glue comes loose.
For what little it's worth - the fore topmast on my little model of H.M.S. Bounty ( http://www.hmsvictoryscalemodels.be/JohnTilleyBounty/index.html ) snapped in half when I was almost finished with the rigging. The spar in question, like all the others in that particular model, is made of degame wood. After I completed the mandatory loud cussing session, took a break, and came back to the workshop in a more-or-less suitable frame of mind, it occurred to me that I had nothing to lose by trying - once - to glue the thing back together. So I put a drop of CA adhesive (which was a relatively new product in the hobby world at the time) on one of the surfaces, took a deep breath, and stuck the other piece onto it. That joint has now held for twenty-nine years. I suspect the key to success in this instance is that almost all the rigging was already in place; the standing rigging was quite effective in squeezing the joint together. If I'd had to put any lateral stress on the joint, it might have given way.
If you do decide to give up on the broken piece, my suggestion for all but the smallest spars is to consider replacing it with wood. It's not as difficult, or time-consuming, as a lot of people think. My guess is that, even allowing for a couple of false starts, once you have the materials and tools in hand it won't take you more than an hour.
Longtime scratchbuilding enthusiasts will tell you that the best woods for spars are slightly exotic hardwoods, such as degame, lancewood, and cherry. They're right - and cherry, in particular, isn't hard to find if you know where to look (for example: http://www.woodcraft.com/family.aspx?familyid=667 ). Hobby shops, hardware stores, and "home centers," like Lowe's and Home Depot, usually carry dowels that will work well, though.
The two common materials for dowels in the US at the moment seem to be oak and birch. Oak is tougher and less flexible, but has a coarse grain; I recommend birch. (That's what the good wood kit companies usually supply. I'm afraid I have no idea what the situation is in Australia; as I understand it that country has all sorts of exotic hardwoods that are good for ship modeling.) Take a careful look at the dowel and make sure the grain runs parallel to the length; if it doesn't, you run the risk that the dowel will snap. Buy plenty; dowels are cheap, and you want plenty of extra ones to practice with. (The standard length in the U.S. is three feet. A couple of those should be plenty.)
It's surprisingly easy to taper a wood dowel if you have an ordinary electric drill. Clamp the drill in a vise, or otherwise fasten it down to your workbench (or a sawhorse, or anything else that will stay put), pointing to your left (assuming you're right-handed. Fold a piece of fine sandpaper into a fairly thick pad. Hold the sandpaper in your right hand. If the spar you're making is longer than a couple of inches, either put a heavy glove on your left hand or arm yourself with a fairly heavy piece of cloth. Your left hand is going to be the "tail piece" of the "lathe" that you've just built.
Cut a piece of dowel somewhat longer than the finished length of the spar, and chuck it in the drill. Turn on the drill at high speed and lock the trigger, so both hands are free. Hold the free end of the dowel loosely in your left hand. (Otherwise the dowel may flex enough to start whipping around and hurt somebody - probably you. You'll quickly find out how tight to squeeze it: tight enough to keep it from whipping, without burning through the cloth.) Hold the sandpaper gently against the spinning dowel, and watch what happens. It probably will take several attempts, but with a little practice you can make mighty nice-looking, accurately tapered spars this way. When the diameter and taper are almost right, switch to super-fine sandpaper to get a nice, smooth finish.
The birch will be almost white in color, but it takes paint and stain well. If this particular spar needs to be painted, whatever paint you're using on the plastic parts will work fine. I like Olympic and Minwax brands, which offer a wide range of stains. The Lowe's store in my town sells little foil-packed samples of stains for about a quarter apiece; two or three of those should be enough for a full set of spars. Finish off with your favorite clear finish. For this particular purpose I like old-fashioned shellac. It's cheap, is easy to apply (with either a brush or a rag), and dries almost immediately.
Hope that helps a little. Good luck.