[bojan]

After ensuring proper ventilation (the water tank rim is elevated ~30cm above ground) there is no problem with condensation any more.
(The water tank I used is not available any more from Bunnings.. it seems the closest alternative could be Gedye compost bin, not sure about the size - 200l should be OK Standard rubbish bin is also OK).

I am also wrapping the plastic bag around the mount (to minimize the amount of air from where the condensation may occur).
It was supposed to be a temporary solution.... Maybe this or early next year (depending on my retirement) I will start building the dome in our backyard in Ballarat property.. then I will simpy dismantle the pier from concrete slab and move it to new location (or make new concrete one).