Petrol prices must drop next week to their lowest level in about two months, South Australia's Royal Automobile Association (RAA) says.

The RAA said pump prices must go as low as $1.28 a liter, reflecting a current fall in prices in Singapore.

"While the value of a barrel of crude oil remains high, motorists and those making media comment need to understand that it is not the sole factor affecting what we pay at the pump," RAA senior analyst Hamilton Calder said.

"Australian oil refineries compete within the Asian region, where the price out of Singapore is used as a benchmark".

"Any change to the Singapore parity price typically takes one or two weeks to flow through to the Australian market, a fact which Australia's major oil companies openly admit to."

In the past eight days, the Singapore parity price has dropped significantly from about $117 a barrel of unleaded petrol to below $107, its lowest level since mid-June