PERTH, Aug. 13 (Reuters) — Australia's thermal coal prices, a benchmark for Asia, fell for the sixth week to languish at an eight-month low of just above $87 a tonne, pressured by growing worries of weak demand in China.

Tepid Chinese demand has been weighing on the Asian and South African coal prices over the past two months and has also dented prices on the forward curves, as traders fear the slowdown in the Chinese economy would dent its import appetite.

Australian thermal coal prices, a benchmark for Asia, started on a downtrend in mid-June and have lost some 13 percent in just two months.

Thermal coal prices on the globalCOAL Newcastle weekly index fell $5.53 from a week ago to $87.09 a tonne free-on-board (FOB) Australia's Newcastle port on Friday.

''The market is terrible, we're clearly seeing a mini-collapse in prices at the moment. Chinese have been very, very quiet and they have been for at least two months now,'' said a producer source in Australia.

''There is a lot of coal floating in the market but spot demand just isn't there. Having said that, its not unusual for prices to fall during this time of the year and to start picking up again in the fourth quarter.''

The combination of rising stockpiles in China, where domestic coal prices are falling, coal output is rising and hydropower supply has jumped, has seriously dented the country's appetite for overseas steam coal.

Apart from trickles of spot demand from China, the Pacific spot market has largely been supported by a number of tenders launched by utilities in South Korea, Taiwan, and Malaysia.