Water early
Early morning is best for watering as leaves have time to dry during the day, preventing mildew growth.
Check your reticulation
Make sure your retic system is working properly and set to the proper watering days and times. It's not much use if the water isn't getting to the areas that need it.
Water deeply
This encourages deep roots which can tap into water resources deep into the soil.
Plants in pots
Water more often and less thoroughly, because water in pots will trickle out of the holes in the bottom. Move them to a shady spot if you can and water twice on very hot days.
Look after budding plants
Budding plants are working hard and need to be kept hydrated. Remember to water them often.
Keep your lawn longer
Longer lawns hold onto moisture so it's easier to keep them looking healthy. Which lawn is best for your garden?
Mulch!
This is a garden's best friend in the hot weather. It shields the soil (and in turn, the plant's roots) from the sun and reduces water evaporation.
What not to do!
Now's not the best time for planting, transplanting, or pruning - save that for the cooler days of autumn. Pruning now could kickstart new growth that might not be ready to face the summer heat.
If you did some planting as the hot weather arrived, protect new seedlings and plants with shade cloth or umbrellas, and water often to establish root growth.