80F (about 27C) is the point at which rabbits potentially become susceptible to heatstroke... 30C is about 86F. The 80F rule of thumb presumes indoor rabbits accustomed to climate control with no precautions/extra cooling measures taken, though - a rabbit accustomed to outdoor life can easily handle summer temps of 15-30C (albeit with mild precautions at the high end of the range).
If a rabbit's been living indoors, they won't have grown the proper coat to live outside in winter or summer, so spring/fall are the times of year to transition a rabbit to outdoor life. A rabbit put outdoors in the spring will be acclimated to outdoor life by summer and will have grown an appropriately light coat.
Between 15-27C, an acclimated rabbit will be totally fine with no extra help (aside from obviously needing most of their area to be in the shade during the hottest part of the day). Between 27-29C, you may want to offer some additional cooling measures if the rabbit seems a bit warm; from 30-32C you *definitely* want to offer some cooling measures. Above 32C, it's best to bring the rabbit inside for the hottest hours of the day unless you have a great cooling set-up for them.
The easiest and best cooling measure is to fill a couple large plastic bottles (like 2L soda bottles) to a few inches below the top with water, then squeeze the excess air out, put the lid on and freeze them - each day (preferably when it starts getting warm, but in the morning would probably be ok if you had to go to work or something), give the rabbit a frozen water bottle to lay against if they get too warm.
A fan blowing across part (but not all) of their enclosure can also help a lot (obviously, it would have to be outside the enclosure so that the rabbit couldn't get to the fan or the cord). Misting their ears with cool water and putting ice in their water bottle or bowl are also very helpful tricks.