What's the weather like in Australia and New Zealand in August?

The season there is just the opposite of ours. August should be winter.

It won't be too cold, it will be above 0 degrees, but it will often rain, so we still need to put on more clothes.

Sweater, sweater. Wear more inside and less outside. Just wear long pants or thin wool pants.

If you swim outdoors, it's up to you. If it is about 0 degrees, you can go into the water if you are not afraid of the cold, otherwise it won't work.