A heatwave is a different problem from an ordinary hot, dry week. A run of days over 30 degrees, especially with hot overnight lows that give plants no real chance to recover, puts stress on a garden fast enough that your normal watering routine won't keep up with it. The moves that help are short-term and specific to the event, not a change to how you garden for the rest of summer.
Shade cloth is the single most effective thing you can put up for the worst of it. A 30-50% shade-rated cloth draped loosely over vulnerable crops, lettuce, silverbeet, new transplants, anything leafy and shallow-rooted, cuts the direct sun enough to stop leaf scorch without starving the plant of light. It only needs to go up for the hottest two or three days and come straight back off once the heat breaks; left on permanently it does more harm than good by weakening growth. An old sheet or shade sail propped on stakes works just as well as a purpose-bought cloth if it's what you've got on hand.
Watering timing matters more in a heatwave than it does the rest of the year. Water early in the morning, before the heat of the day sets in, so the moisture has time to soak down to the roots before it's lost to evaporation. Watering in the middle of the day wastes a large share of what you put down and can scorch leaves that get splashed in direct sun. A heavy, thorough soak that wets the soil well below the surface will get a garden through a heatwave far better than several light sprinkles through the day, since shallow watering evaporates again almost as fast as it goes on.
Mulch works as insulation as much as it works to retain moisture, and during a heatwave that insulating effect is worth topping up specifically. A generous layer, 5 to 7 centimetres of bark, pea straw or compost, keeps the soil noticeably cooler and slows how fast it dries out compared with bare soil sitting exposed to full sun all day. If your beds are already mulched at a normal depth, it's worth adding an extra layer just for the duration of a forecast heatwave rather than assuming what's already down is enough.
Containers heat up and dry out far faster than garden beds during a heatwave, because a pot heats through its sides as well as its top and holds a much smaller volume of soil to buffer that heat. Grouping pots together so they shade each other, and moving anything portable into afternoon shade for the worst of the event, makes a real difference. A pot left in full sun on a concrete patio through a heatwave can dry out within hours of being watered.
Knowing what a plant's wilting actually means saves a lot of unnecessary panic watering. A plant drooping in the full heat of the afternoon and looking perfectly upright again by evening is showing a normal heat response, not a sign it's dying of thirst, and dumping more water on it at that point usually doesn't help. A plant that's still wilted the next morning, before the day's heat has even started, genuinely needs water. Learning to tell the two apart is more useful during a heatwave than watering on instinct every time the garden looks stressed.
A plant drooping in the full heat of the afternoon and looking perfectly upright again by evening is showing a normal heat response, not a sign it's dying of thirst, and dumping more water on it at that point usually doesn't help.