The honest answer is to leave the dark three-piece suit at home. Four degrees south of the equator, on sand, in the late afternoon, a heavy wool suit is hot, it photographs stiff, and it will have the groom miserable by the time the bride arrives. The grooms who look best in our films are almost always the ones who dressed for the climate on purpose, in light fabric and light colour, and looked relaxed because they were.
Fabric and colour first
Choose linen or a light cotton blend over wool, and a lighter colour over a dark one. Beige, stone, pale blue, soft grey and white all catch the island light and stay cooler, while black and charcoal absorb the heat and read heavy against bright sand. A linen shirt with the sleeves rolled, tailored light trousers, and at most an unstructured linen jacket you can take off for the ceremony is the formula that works. It looks intentional, not underdressed.
Feet, and the barefoot question
Most grooms go barefoot for the ceremony, which is the whole point of a beach wedding and photographs wonderfully. Keep a pair of good leather sandals or espadrilles for walking to and from the sand and for the evening. Avoid brand-new shoes that will fill with sand and rub, and skip socks with the sandals, which the island light is unforgiving about.
Bring it from home
As with the dress, buy the outfit at home where you have the choice and the time to tailor it, and bring it with you. Victoria has shops but not a reliable formalwear selection, so this is not something to leave to the island. A light shirt packs flat and creases fall out in a steamy bathroom overnight, the same trick that saves the gown.
What the season is actually doing to you
The heat is only half of it. From roughly May to September the south east trades run steady and dry, often ten to twenty knots, so a jacket that behaves perfectly in a still room becomes a sail on an exposed beach. From November to March the wind drops but the humidity climbs, and January and February are the wettest and heaviest months of the year. Those are two completely different problems.
In the windy months, choose weight over structure. Linen that is slightly heavier hangs instead of flapping, and an unlined jacket you can take off for the portraits is worth more than a beautiful one you cannot. In the humid months, choose breathability and accept that you will be photographed in shirtsleeves at some point, because everyone is. Either way the temperature sits near thirty degrees all year, so wool is not a plan.