Governing body World Aquatics delayed the race because water quality was “exceeding acceptable thresholds”.

The women’s 10km race had already been postponed on Tuesday and moved to Wednesday.

When it did finally get under way at 4:00 pm (0800 GMT), Moesha Johnson added world championship gold to the silver medal she won at the Paris Olympics last year.

The Australian came home first in 2hr 7min 51.30sec, finishing ahead again of Paris Olympics bronze medallist Ginevra Taddeucci of Italy (2:07:59.70).

Lisa Pou of Monaco (2:07:57.50), who was 18th in the Paris Olympics last summer, came in third for a surprise bronze medal.

World Aquatics said early Wednesday morning that the new afternoon time slots for the races were “subject to acceptable test results”.

But soon after, they confirmed both races would proceed in the afternoon as further analysis had found “significant improvement” in bacterial content and water temperature.

“Regular water quality monitoring and testing continues throughout the competition period to ensure athlete safety,” World Aquatics said in a statement.

“The health and safety of all participants continues to guide all competition-related decisions.”

The governing body said levels of E. coli bacteria had dropped to between the ranges of “good” and “excellent”, according to World Aquatics and World Health Organization regulations.

Water quality was a big problem for events held in the River Seine at last year’s Paris Olympics.

Of the 11 days of events and training scheduled in its murky waters, only five got the green light.

The river remained dogged by pollution problems despite a 1.4 billion euros ($1.5 billion) upgrade to improve the Paris sewerage and water treatment system.