Australia's veteran opener David Warner hit a double century on Tuesday against South Africa in his 100th Test before being helped off the Melbourne Cricket Ground in agony.

En route to three figures, the 36-year-old passed 8,000 Test runs -- the eighth Australian to do so -- after coming into the game under pressure after a lean run.

He notched his first Test century since January 2020 to prove the doubters wrong on day two of the second Test and with the opposition bowlers tiring in searing heat, he battled the elements and converted it to 200.

His third Test double century came off 254 balls with 16 fours and two sixes in a test of endurance in the heat.

He needed treatment for cramp several times and eventually retired in pain on 200, helped off the field.

Warner, who dropped to his knees which he reached 200, insisted before the match that "you're never out of form".

"They're not the words that I use and they're definitely not used in our change rooms," he said. "It's about (being) out of runs."