I am also not sure about the Australian aspect, but if in the US, I would:
Put $100,000 in StreetShares Veteran Bond. It pays 5% interest accumulated daily (and has a bonus of $1000 for 100k). If you withdraw before 12 months, you face a 1% fee (essentially the bonus). At 6 months, that is 2.5% return on your investment and 12 months is a 6% return.
I would put the other $200k into Motif Investing. Scroll through the community Motifs and find a low volatility bond or stock mixture motifs to your liking.
You could also set some aside to make your home more energy efficient (solar panels, heater, etc.)
If it truly is long term investments you are looking for, you could always buy parts of the New Zealand forest for lumber yield in a decade or two.