Welcome to the Bunnings Workshop community. Great to see that you have already got lots of value from other members' posts. Feel free to post anytime you need a hand or have a project to share.
I'm sure @EricL and other helpful members like @tom_builds will be happy to assist as soon as possible, but in the meantime, have you got a plan of what you want to build that you could share with us? I'm sure it would help members to be in a better position to recommend the right timber to use.
Many thanks, we are looking forward to seeing your pergola project come to life.
Thank you for joining us and sharing your question about what timber to use for the pergola posts and beam.
The Boral 120 x 45mm 3.6m F27 KD Solid Seasoned Structural Hardwood is a seasoned structural timber and is ideal for all structural applications including sub-floor construction, stair components, highly loaded truss members, wall lintels over openings, and high strength structural beams. However, I will have to ask our provider if it will work for inground applications.
Would it be possible for you to post a photo of your plan and where you plan to anchor it? This will give us a better idea of what you would like to build and allow us to give you a more comprehensive recommendation. We look forward to seeing your plans.
Here is a handy link to give you ideas: How to build a pergola
We look forward to hearing from you.