The survey also indicates supply from top world exporter Saudi Arabia is climbing after a cutback earlier this year.
Opec meets on Monday in Vienna to set supply policy and is expected to hold its formal output limit unchanged as oil prices remain near $70 a barrel.
August?s supply was the highest since 29.99mn bpd in November 2005, according to Reuters surveys.
Opec produces more than a third of the world?s oil and members except Saudi Arabia are pumping at full tilt.
Nigerian exports rose in August as output climbed from the offshore Bonga oilfield and as a pipeline leak on the Bonny field had less of an impact on exports as expected.
Royal Dutch Shell declared force majeure on Bonny exports in July after the leak, which shut around 210,000 bpd on July 21 until Shell said pumping resumed on August 10.
Venezuelan supply also climbed as maintenance was completed at heavy oil processing plants, which turn tar-like Orinoco crude into oil that refineries can process.
The survey puts output from the 10 Opec members that agree to output quotas, all except Iraq, at 27.88mn bpd in August. They have produced below their 28mn bpd target for every month this year.
Iraq?s oil exports fell 60,000 bpd in August to 1.64mn bpd as sabotage attacks stopped the flow through its northern pipeline to Turkey, shipping sources said.
When the pipeline to Turkey is down, Iraq relies on its southern Basra terminal to ship oil to international markets.
Supply from Iran averaged 4mn bpd in August. Output was about 4.1mn bpd in July, higher than earlier estimated, as Tehran stepped up sales of oil from storage.
Output in Indonesia slipped again to the lowest in more than three decades.
