Hi Markass,
There are no stingers in Cairns in July. They (Box Jellyfish and Irikanjis), are prevalent from October to May, even then, when the wind blows up, the stingers go to the bottom and so you are still pretty safe with a basic stinger suit.
July is sweet as, just go and get some wind, don't freak over the jellies. They usually drag nets to see how many are caught at the end of the season (May) and mostly they catch the odd one in May but June is fairly clear and July is definately the go.
Kite with confidence!
Good winds,