Rio de Janeiro has come under fire for its water quality once again after canoeists complained that weeds made an Olympic test event more like an obstacle course.

Austrian Viktoria Schwarz finished in fourth place in her race on Saturday but criticized the Rodrigo de Freitas lagoon, where canoeing and rowing events will take place at next year's Rio de Janeiro Olympics.

"The course is beautiful but the water is full of weed," she said in an Instagram post. "So it felt more like a 500m with resistance."

The post was accompanied by a photo of her rowing partner Ana Roxana Lehaci fishing clumps of flora from the water.

The pair finished fourth in the K2 500m women's race, one of six finals contested on Saturday.

Portugal's Fernando Pimenta, who won the K1 1000m event, also complained about the conditions.

"Unfortunately, the algae is hampering some athletes' performance and that's not good for the sport," he said in comments reported in the Folha de S.Paulo newspaper.

The water quality in Rio de Janeiro, particularly in the sea waters where sailing, swimming and triathlon events will take place, has come under heavy fire after the government admitted it will not clean up the water as it promised.

Independent studies showed the level of bacteria and viruses in the water far exceed international norms.

The canoeing and rowing events in the city's lagoon have also been questioned, with dead fish regularly washing up on the shores.

Brazil played down the latest criticism, with organizers saying such problems were normal the world over.

"This is a common occurrence in our sport," said Simon Toulson, the secretary general of the International Canoeing Federation. "It is not going to happen in the Olympic Games because we have a long time to prepare and we are going to clean up the course."