My seedling's an apple plant. I'm in australia and it's winter here. At night temperatures can drop to around 50 f but during morning around 10am and evening temperatures can be quite warm and go as high as 68+ f. I don't think I can leave my plant inside because it's too humid and it doesn't like it. So will it be ok to leave it outside?

I say yes, as long as you acclimate it to out doors, especially the sun

Ok thanks. (I meant to say 'afternoon' instead of evening). I'll keep a close eye on it.

Welcome to NGA, @user468 !

LOL, I have to love people (wherever they may be on the planet) who think 50-68F is "winter." (that's not much cooler than my 'summer' !)

Unless it is some sort of decidedly un-hardy, tropical type of apple tree, it should be perfectly fine outdoors... they survive and grow here where I live, through -30F winters.