* Rates from Western Australia to China fall from six-month high
* Capesize fleet to grow 11.6 pct -Bancosta
By Keith Wallis
SINGAPORE, May 19 (Reuters) - Freight rates for large capesize dry cargo ships on key Asian routes could slide next week as charterers rein in their activity following a flurry of fixtures which pushed rates from Western Australia to China to a six-month high this week, ship brokers said on Thursday.
That came as all three Australian iron ore miners - Rio Tinto, BHP Billiton and Fortescue Metals Group - cashed in on higher iron ore prices with a raft of fixtures this week. Iron ore prices have climbed from a low of $37 a tonne in December to around $56 a tonne this week.
"There were two days of optimism where the market was pushing up and everything is looking rosy, then the market comes off and rates fall through the floor," said a Singapore-based capesize broker on Thursday.
"The capesize market is a never-ending roller coaster. Rates are up, then they're down. There's no sustainability," the broker said.
Capesize freight rates for a spot cargo from Western Australia to China climbed to $4.63 a tonne on Tuesday, the highest level since Dec. 1, 2015.
But they dropped again from Wednesday.
"The index level is now $4.35 a tonne but it's going to come down. Offers from charterers are now around $4.20/$4.25 a tonne so rates are going to get smashed," the broker said.
Iron ore and coal are staple cargoes for capesize ships which can carry around 170,000 tonnes of the commodities.
"It does seem that the Australian miners are trying to sell as much (iron ore) as possible while iron ore prices are good," Norwegian ship broker Fearnley said in a note on Wednesday.
"But all the miners expect iron ore prices to drop to considerable lower levels in months to come," the Fearnley note added.
The dry cargo market is facing continued pressure from significant oversupply of ships and marginal trade growth.
The capesize fleet is estimated to grow by 11.6 percent from now to 2019 with around 30 million deadweight tonnes (dwt) scheduled for delivery this year, compared with just one percent growth this year in seaborne cargo demand, shipbroker Banchero Costa said in a research report on Thursday.
Freight rates for the Brazil-China route rose to $8.46 per tonne on Wednesday, against $7.39 per tonne on the same day last week.
Capesize charter rates for the Western Australia-China route climbed to $4.36 per tonne on Wednesday compared with $3.51 per tonne a week earlier.
Charter rates for smaller panamax vessels for a north Pacific round-trip voyage were higher at $4,829 on Wednesday, the highest since March 3, from $4,583 per day a week earlier.
Freight rates in the Far East for smaller supramax vessels were again steady this week holding around $5,000 per day for a Singapore to China voyage, Fearnley added.
The Baltic Exchange's main sea freight index rose to 642 on Wednesday from 652 last week. (Editing by David Evans)