It's an open air coop, not closed so it might be cooler than you think. I'll check the coop thermometer - ok, it says 11C on the coop thermometer right now (4.50pm) and the Weather Bureau says the temperature is 11.2C, so that's quite close. However, the hens will create warmth on the roost once they've gone to bed.If its 12C outside, then the temp in the coop will be higher.
I know (from others) that a steam device does the job. Getting boiling water in the coop in another way seems difficult.
For boiling water, I can bring the roost perches to the kettle in the laundry. The rest of the coop is made of metal, including the sawhorses on which the roost perches are mounted. The floor is sand covered with wood shavings, so that's a risk factor.
I'll try to establish a routine of pouring boiling water onto the roosting perches every Saturday. Assuming there are mites in the coop, that should keep them in reduced numbers.