The government projection is out of step with some private forecasts, with UBS and Citi calling for iron ore prices to average around $64 a tonne in 2018 - flat on 2017's $64.30 - with the market proving suprisingly resilient.
Spot iron ore, currently around $75 a tonne <.IO62-CNO=MB>, last traded below $52 in June 2017, but Department of Industry, Innovation and Science resource and energy analyst David Thurtell pointed to an expected contraction in China's steel industry.
"We're still comfortable with where our forecast sits," he said.
The world's top three mining companies, BHP (>> BHP Billiton Limited) (>> BHP Billiton Plc) and Vale (>> Vale SA) rely heavily on iron ore sales for the bulk of their revenue despite efforts to diversify more into other industrial raw materials, such as copper, aluminium and coal.
Brazil-based Vale is planning to lift iron ore exports 7 percent in 2018 to 390 million tonnes. In Australia, Rio Tinto and BHP, along with Fortescue Metals Group (>> Fortescue Metals Group Limited) aim to add about 170 million tonnes of new capacity over the next several years.
The forecast price decline will continue into 2019, when the steelmaking raw material will average only $49 a tonne, the department said in its latest commodities outlook paper.
"The iron ore price is expected to experience some ongoing volatility in early 2018, as the market responds to uncertainty regarding the impact of winter production restrictions on iron ore demand," it warned.
The lower prices will reflect growing supply from low-cost producers and moderating demand from China, it said.
China is in the process of closing ageing, high-polluting steel mills and induction furnaces to curb overcapacity in the sector.
China's President Xi Jinping said in October that fighting pollution was one of the country's key tasks through 2020.
Australia's liquefied natural gas (LNG) exports are forecast to climb to 76.5 million tonnes in the year to end-June 2019, from 63 million tonnes forecast for the 2017/18 fiscal year and 52 million tonnes last year.
Between 2016/17 and 2018/19, LNG should add A$14 billion ($11 billion) to Australia's export earnings, while iron ore is forecast to subtract A$10 billion, according to the department.
The shift follows the construction of $180 billion of new gas projects. The rise in LNG earnings will be underpinned as three remaining projects under construction hit their stride, it said. These are Chevron Corp's (>> Chevron Corporation) Wheatstone project, Inpex Corp's (>> Inpex Corp) Ichthys and Royal Dutch Shell's <RDSa.L> Prelude.
Prices for coking coal, another key steel-making ingredient, are forecast by the department to drift lower over the next eighteen months from last quarter's benchmark price of $192 a tonne as rising supply more than offsets demand.
It also expects thermal coal prices to ease through 2018 and early 2019, with the Newcastle spot price <GCLNWCPFBMc1> forecast to drop 12 percent to an average $77 a tonne in 2018, and by a further 6 percent to $70 in 2019.
($1 = 1.2719 Australian dollars)
(Reporting by James Regan; Editing by Christian Schmollinger and Richard Pullin)
By James Regan