The outlook for nickel weakened over the December quarter, with its forecast downgraded.

Nickel has been downgraded from $13,500/t to $12,000/t for next year, and from $14,000/t to $12,500/t for 2020.

It should rise to $13,000/t in 2021.

S&P said the nickel outlook reflected the forward curve and its views that prices would continue to be depressed on the grounds of trade tensions between China and the US.

The 2019 outlook for copper remains at $6100/t, with 2020's forecast increased from $6200/t to $6300/t before a lift to $6500/t in 2021.

"We raised our mid-term copper price assumptions toward $6500/t because we believe those price levels address well medium-term fundamentals and risks," S&P said.

The expected 2019 price for zinc was lowered by $100/t to $2700/t, and from $2800/t to $2600/t in 2020.

"The revision mainly reflects our expectation for higher industry supply amid relatively tepid demand growth," S&P said.

S&P was far more bearish on base metals that the Australian government's Office of the Chief Economist, which published its latest figures just before Christmas.

Iron ore is expected to average $65/t, up from $60/t, next year, before dropping to $60/t in 2020.

S&P said the forecast reflected its expectation of balanced market fundamentals and steady demand.

The three-year outlooks for aluminium and gold were left unchanged at $2100/t and $1250 an ounce respectively.