Hi @cindex,
Thank you for your question and welcome to the Bunnings Workshop community, it is fantastic to have you with us.
Is it possible to get some photos of the rotting timber? It would be very difficult to give you advice without seeing the pergola.
You will need to use timber that is suitable for outdoor use such as H3 Pine. The specific size of timber would likely be decided by the rest of the structure.
You might like to have a look at How to choose the right timber for the job for some further information on selecting timber.
Once you've uploaded some images, I'll offer further advice on specific methods for replacing the timber.
Let me know if you need a hand uploading images, I'm happy to help.
Jacob
