If you own a hammer, then it is tempting to suggest that the simplest option is to work out what height of wood would do the job, buy a replacement piece and appropriate sized nails from bunnings or equivalent, knock the old one off, and hammer the new one on. The issue with that is that getting the right size might be difficult. Termite treated blue pine appears to come in 35mm and 45 mm options, but is sold in linear metres for a few $ a metre. Other variations appear to also be 35mm or 45 mm options.

Hopefully someone more helpful than me sees your question. If you are really stuck, I can ask artisanat for suggestions, and maybe they would be able to help with the actual repairs.