Rio Tinto says its second quarter Pilbara iron ore shipments have fallen about 3.0 per cent during a “challenging” period hurt by clean-up works from Tropical Cyclone Veronica.

Shipments from the miner’s Australian operations fell to 85.4 million tonnes in the three months to June 30, compared to about 88.5 million tonnes in the same period a year ago, Rio Tinto said on Tuesday.

The result was a 24 per cent improvement on the mining giant’s first quarter shipments, which had been hampered by the disruption of Cyclone Veronica late in March.

Chief executive Jean-Sebastien Jacques said the past six months had proved a challenging operational performance.

“Whilst we experienced operational and weather issues at our iron ore operations in Australia, pricing and market demand has remained robust,” Mr Jacques said.

Rio Tinto said its annual iron ore shipment guidance remained at 320 million tonnes to 330 million tonnes following last month’s operational revision.

The miner’s Q2 iron ore production also dipped, falling 7.0 per cent to 79.7 million tonnes compared to a year ago.

First half production is down 8.0 per cent to 155.7 million tonnes.

Meanwhile, the company said mine design complications at its Oyu Tolgoi underground copper project in Mongolia could delay first sustainable production by up to 30 months, potentially out to June 2023.

Preliminary cost estimates at the project have now increased by between $1.2 billion and $1.9 billion, with the total cost now estimated between $6.5 billion and $7.2 billion.

Rio Tinto said on Tuesday it was reviewing the carrying value of its investment in the Oyu Tolgoi project, and will announce if any changes are required when it reports its half year results on August 1.