I think the best way to house axies in a stable cool environment during summer without the use of a chiller, is to fridge them.
Any other cooling methods tend to be temporary and can only reduce the temperature by a couple of degrees. For those hot Australian summers reaching up to 40+ degree celsius, coupled with the humidity, it is a rarity to be able to even keep the tank below 25 regardless of cooling methods used.
For long term management/cooling of tanks, i would recommend you use a mesh top screen lid to allow evaporative cooling. That alone can get the tank temperature down by 2 degrees. Location of the tank is also important. If you live in a house with double or triple levels, it is best to place the tank at the lowest level of your house (ie basement). The lowest level is normally the coolest. Also avoid strong heat and light exposure.
You can also try directing a pedestal fan across the tank water surface to accelerate evaporative cooling. This is effective only if the humidity level is low though. Air conditioning the room is another method.
Ice bricks can evert an emergency but in my opinion too laborious, too temporary and result in temperature fluctuations. It may not be the most practical method.