To ask, do you have an early weigh-in on the day of competition? A lot of competing powerlifters I've known have been able to get rid of up to 5kg in the last day or two prior to competition and then rehydrate in time for lifting, so the longer the gap between the weigh-in and the start of the competition will make the difference for how to go about it.
Some people rely on diuretics (which can be risky), but otherwise, I've known a few people who do extreme sodium restriction at least 2-3 days prior to competing, they take stimulants (or, at least, they'll drink LOTS of caffeinated coffee), and go low carbohydrate until weigh-in. It's not as easy to cut sodium in a hurry sometimes unless you've done plenty of checking on what would fit the bill, but I find that for myself, plenty of strong coffee and keeping carbs really low can easily take off 1.5-3kg over a day for me if I'm strict. Unsalted nuts, plain tofu or seitan, salads with low sodium dressing, a tablespoon of flax seed oil once in a while and distilled water are the sort of things that I find tend to keep the water off in conjunction with the caffeine if I need to take in calories but don't want any water retention to go along with it.
Of course, there's also the option of wearing a plastic "sweat" suit underneath your clothing for a day prior to competition or spending a good amount of time in the steam room to speed things up, but always remember, the more you mess with removing large amounts of water QUICKLY, the more you can mess with your health. I'd plan to start working toward the water reduction 3-4 days out so you aren't scrambling to make weight on competition day, but perhaps some of the other bodybuilders from here can offer tips that would help as well!
"A 'hardgainer' is merely someone who hasn't bothered to try enough different training methods to learn what is actually right for their own damned body." - anonymous