The Reserve Bank's Index of Commodity Prices dropped by a preliminary 6.8 per cent on a monthly average basis in May.
And this followed on from a revised 4.4 per cent decline in April.
Despite the big decline in recent months, the index is still 32.6 per cent higher over the past year in SDR terms (and 27.1 per cent higher in Aussie dollar terms).
The annual gains have been driven by the prices of bulk commodities, namely coal and iron ore.
However, iron ore prices have been slammed in recent trades, so monthly average prices likely have a good way to fall yet.