News: UPDATE 1-Rains cut Rio Q1 iron ore output 3 pct

• Rio Q1 Australia iron ore output slips
• Q1 shipments steady 76.7 mln tonnes vs yr ago
• Cuts mined copper guidance

(Adds iron ore, aluminium, copper figures, quote)

SYDNEY, April 20 (Reuters) - Global miner Rio Tinto (RIO) on Thursday said first-quarter iron production from Australia fell 3 percent from the same period a year ago due to wet weather at its mines, but kept its full-year guidance intact despite weakening ore prices.

Pilbara mines output totaled 77.2 million tonnes, the company said.

Full-year shipping guidance was kept at 330 million-340 million tonnes.

Shipments from the Australian mines in the first quarter were flat at 76.7 million tonnes aginst the year-ago period, but down 13 percent from the previous quarter.

Ship loading was impacted by a cyclone, with parts of its rail line hit by heavy rainfall.

"Despite these disruptions, shipments were in line with the first quarter of 2016 and guidance for 2017 remains at 330 to 340 million tonnes," the company said.

Rio Tinto and rivals Vale , BHP Billiton (BHP) and Fortescue Metals Group (FMG) are facing a rapidly declining iron ore price amid waning demand from China, the biggest market for ore.

The worldwide iron ore surplus reached 70 million tonnes last year - more than total U.S. consumption last year - and could balloon to 90 million tonnes in 2017, according to Citigroup.

Iron ore prices are down more than 33 percent since a mid-February peak of $94.86 a tonne and forecasters are warning of a further pullback.

Australia’s Department of Industry, Innovation and Science predicted iron ore prices would backtrack to U$55 a tonne in the fourth quarter.

Next year’s forecast calls for iron ore prices to reach $51.60 a tonne, according to the department.

In other minerals, Rio Tinto stuck to a full-year target of producing between 3.5 million 3.7 million tonnes of aluminium following a 2 percent rise in first-quarter production.

But mined copper guidance was reduced to 500,000-550,000 tonnes from as much as 665,000 tonnes as a result of a strike at the Escondida mine in Chile and the curtailment of production at the Grasberg mine in Indonesia.

Refined copper production guidance remains unchanged at 185,000 to 225,000 Rio said.