Investors who took money out of the ASX ahead of the first debate for the US presidency were vindicated as the bourse finished further down in its wake.

The S&P/ASX200 benchmark index was lower before the Donald Trump and Joe Biden stoush, then slipped more and closed lower by 136.2 points, or 2.29 per cent, at 5815.9 points on Wednesday.

The index recorded its first monthly decline (4.04 per cent) since the 21.18 per cent crash in March when the coronavirus pandemic gripped Australia.

The ASX200 lost 1.39 per cent for the quarter.

The All Ordinaries index finished Wednesday down by 132.2 points, or 2.15 per cent, at 6009.3.

The Aussie dollar was buying 71.10 US cents at 1633 AEST, up from 70.81 US cents after the close of trade on Tuesday.