(Adds Cadia mine output figures, CEO comment and production...

(Adds Cadia mine output figures, CEO comment and production costs)

Oct 26 (Reuters) - Newcrest Mining Ltd (NCM) said on Thursday its first-quarter gold production slid 15 percent from the year-ago quarter and copper output slumped nearly a third as repairs were ongoing at the company's flagship Australian mine which was damaged by an earthquake.

Australia's largest independent gold producer said on Thursday its total gold output from mines in Australia, Indonesia, Papua New Guinea and Ivory Cost fell to 522,917 ounces in the quarter ended Sept. 30, from 615,498 ounces in the same period a year earlier.

It maintained fiscal 2018 gold production guidance of between 2.4 million and 2.7 million ounces.

The Cadia mine produced 120,514 ounces of gold in the September quarter, down from 195,301 ounces a year ago. Operations have been gradually restarting at the mine since mid-July.

"While gold production for the group was lower than the prior quarter, we conducted a number of major plant shutdowns across the group in the September quarter, and we expect gold production to increase over the coming quarters," Newcrest Managing Director Sandeep Biswas said in a statement.

The decline in output drove Newcrest cost of production to $898 per ounce compared with $790 a year ago, on an all-in sustaining basis.

The company's overall gold production was also below a UBS forecast of 538,500 ounces.