Tasmanian ‘oak’ is a marketing name that includes three similar timbers, alpine ash, mountain ash and messmate. All three fall into Group 3 as confirmed individually by our Regulatory Information Report 41117.11. A copy of RIR 41117.11 can be downloaded from our website via this link https://www.woodsolutions.com.au/resources/standards-codes/fire-test-reports.

We are not familiar with the problem you describe. It should be possible to sand Tas oak (even 'aggressively') without causing colour change. Tas oak normally varies in colour from straw, to pink, to reddish brown as illustrated here https://thetimberline.com.au/pages/tasmanian-oak. The timber in your image looks very dark so perhaps sanding it has generated enough heat to darken the wood. We can't suggest a remedy, other than turning the treads over, so the darkened wood is on the underside. Sorry we can't be more help.

Unfortunately WoodSolutions Australia is not able to help you with this very worthy project. Our role is to provide advice about wood products. We do not trade in timber or plywood.