I would like to start a new batch of mead. However currently the air temperatures where I live can reach 30 degrees (Centigrade) and I have no air conditioning at home (so the temperature at home can be about 26 degrees Centigrade). Is it OK to start brewing now, or is it better to wait till autumn?
It depends somewhat on what flavors you are looking for and how long you want to wait, post-fermentation, to drink it.
Warmer fermentation is going to produce more fruity esters from the yeast, but they also produce more complex (hot) alcohols. Primary fermentation will finish relatively quickly, but the mead is going to have to sit in secondary for possibly a year or more for those hot alcohols to break down. More of your end product mead's flavor will be from the yeast.
Cooler fermentation is going to produce a cleaner flavor, both as far as the esters and the alcohol. Primary fermentation will take a little longer, but you may have a drinkable product in 6 months. A cooler fermentation's flavors will also be more about the flavors from the honey than flavors from the yeast.
I would wait until you can keep the fermenter at 20C (68F). You can do this now, if you have an insulated cooler large enough for your fermenter and some plastic bottles that you can freeze and rotate into the cooler.
To a large degree it depends upon the yeast. On one extreme is something like EC-1118 which can handle warmer temps without much of a problem (producing a very dry mead), on the other is something like D-47 (creating a more semi-sweet traditional mead) which (from what I've heard) will start kicking out a lot of fusel alcohols at those temps. Those fusels will take a lot of aging to smooth out, if ever.
That being said, if you can wait you probably should, since any yeast will have more of a risk of producing off-flavors as the temperature rises. Consider setting up a swamp cooler, which is basically just a big tub of water that you submerse your fermenter in, keeping the temp down with ice packs until primary fermentation is over.