Starting on Thursday, the testing is expected to go for one week.

The stack was damaged in a fire on October 13.

Bluescope general manager John Nowlan said the testing would involve circulating char and heating the regenerator up.

''When this activity commences there may be some visual emissions from the new steel stack caused by water vapour and some sulphur oxides condensing in the atmosphere,'' Mr Nowlan said.

''Emissions testing during the interim arrangement have been well within licence limits.''

He said the company were on track to commission the new steel stack within the indicated 10-week period.