A cyclist stopped to give water to a koala in Adelaide in a viral video filmed on Friday as a heatwave continued to grip South Australia.

Anne Heusler was cycling in the Adelaide Hills with a group of other cyclists when she stumbled on the koala, who climbed right up onto her bike as she gave it water.

Video obtained by Reuters showed the cyclists bemused by the koala as it drank heartily.

"After we gave him all the water we had from the bottles of about 7 cyclists, we escorted him back off the road into the bush," Ms Heusler told Reuters. "I had to ride home thirsty!"

Adelaide has been gripped by a heatwave along with much of Australia, with temperatures in parts of South Australia reaching above 40C on Friday, while Victoria's capital of Melbourne is forecast to hit 43C on Monday.