Why not show the landlord the beauty of the heavens above - he / she might fall in love with it, you never know...
The "Bunnings observatory" should be easy to disassemble and removable without a trace, so where is the problem for a landlord ? I wouldn't do a concrete pier without speaking to them first, though.
We're renting as well, big house with 13 acres of land, cats & dogs & horses & fish tanks, and do all sorts of funny things - and our landlords love it each time they come; over time we're actually developing a really nice relationship. What we do is we ask, and we show what and why we want to do certain things - so far it has always worked out fine.
BTW, I'm German, so where I come from renting is the norm, and buying is being done once in your life only - it's exactly the other way round as down under. And tenants are no outcasts...

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Cheers, Florian