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Welcome to the Bunnings Workshop community @geraldv. It's sensational to have you join us, and many thanks for your question about timber gradings.
Everything above ground needs to be H3-treated, and every in contact with the ground or below it needs to be H4-treated. Have a look at this helpful guide on How to choose the right timber for the job.
In the case of the decking, you might like to use H3-treated Pine or a hardwood like Merbau.
Please let me know if you have any questions.
Mitchell
