Kiwi race walker Quentin Rew needed the help of race officials after collapsing after the finishing line in the men's 50km event on a sweltering day in Rio.

The 32-year-old appeared to cramp up after crossing the line in 12th and hung on to the race barriers while course attendants covered him with a towel and poured bottles of water over his head.

Rew came close to beating his personal best of three hours 48.48 second for the distance, clocking 3:49.32.

Australian Jared Tallent looked on track to win gold, only to be overtaken by eventual winner Matej Toth from Slovakia at the beginning of the last of the 25 laps.

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The Australian, who claimed silver, was belatedly awarded the gold medal from the 50km race at the 2012 London Olympics in June this year after Russian Sergey Kirdyapkin was disqualified for doping. Japan's Hirooki Arai was third.

Rew, who was disqualified in the 20km event earlier at the Games, gradually picked his way through the field after being 26th through the opening 15km.

He was 15th by the halfway stage and moved into 12th with 15km to go. Rew was 30th over the distance at London four years ago.

-AAP