Some major listed companies have been re-rated by broking houses this week and investorTV's Lauren Groves finds out why.

Alumina Limited was upgraded to a "buy" this week, with rising commodity prices pushing up the bauxite miner and smelter’s earnings forecasts.

Merrill Lynch raised Alumina Ltd’s earnings predictions by 1 per cent, 30 per cent and 62 per cent respectively in 2007, 2008 and 2009.

However Merrill Lynch said aluminium prices would be affected by a strong Australian dollar and increasing production costs.

Major steel products maker and iron ore miner OneSteel also benefited from higher resource prices.

Merrill Lynch said OneSteel was a stock to watch, predicting current fiscal 2009 forecasts will be revised upwards.

The broker raised its fiscal 2009 earnings estimates for OneSteel, due to a rise in iron ore prices and increased project profitability.

Goldman Sachs JBWere has upgraded Sydney-based gas company APA Group from hold to buy.

The re-rating is based on expectations the company will not perform capital raising activities this year.

Goldman Sachs said the recent drop in APA’s share price provided a good opportunity for potential shareholders.

The broker also said APA was well-positioned to take advantage of increasing gas quantities on Australia’s east coast.

Finally, Origin Energy has been downgraded from buy to hold.

Goldman Sachs downgraded Origin on the back of competitive threats to retail margins and its recent share price rise.