The problem you describe is almost certainly 'iron stain'. The US Forest Service's Forest Products Laboratory has an informative data sheet on iron stain that can be downloaded here: https://www.fs.usda.gov/treesearch/pubs/48449. It's not a problem specifically with spotted gum, but can occur with any wood that has a significant tannin content. It's often difficult to trace the source of iron - it can be iron filings from adjacent metal work, metal particles from fasteners and so on. If there is a source of iron on site perhaps the explanation for the lack of staining on the western deck is simply that it doesn't get wet. The reaction only occurs in the presence of moisture.Iron stain
Answered on 11-09-2020
Please note that our answer is based on the best advice available at the time. If the National Construction Code, Australian Standards or local requirements have been subsequently amended, our answer may no longer be correct in all details. For more information, please read our disclaimer.