SYDNEY, Oct 3 (Reuters) - Iron ore exports to China from Australia's Port Hedland, which handles about a fifth of the global seaborne iron ore market, rose 3.2 percent in September from August, underscoring the grip Australia has on the world's biggest steel market.

Year-on-year, September shipments of just under 23 million tonnes were up 52 percent, data released by the Port Hedland Port Authority show.

Overall, iron ore exports from the port were 29.0 million tonnes, up 5.8 percent from 27.4 million tonnes in August. Total shipments were up 46 percent year-on-year.

Australia's Bureau of Resource and Energy Economics on Wednesday forecast China's 2013 iron ore imports would rise 12 percent to 831 million tonnes from this year.

Port Hedland is used by BHP Billiton , Fortescue Metals Group and Atlas Iron to ship iron ore cargoes, which are expected to exceed 200 million tonnes this year.

Shipments to Japan were steady month-to-month at 2.2 million tonnes, while shipments to South Korea increased to 2.9 million tonnes in September from 2.1 million in August.

The Port Hedland figures do not include iron ore shipments by Rio Tinto , Australia's biggest producer.

Rio Tinto is expanding its export capacity to 290 million tonnes from 220 million from Indian Ocean ports south of Port Hedland on Australia's west coast.

It is due to decide by the end of 2013 on whether to push ahead with a further expansion to 360 million tonnes.

Brazil is the world's second-largest exporter of iron ore, supplying about 30 percent of world trade in the steel-making raw material.