i did a lot of research on this a while back. still gunning to do this and great to hear bunnings has a kit!
regarding the bowing, if you :
keep the pond under 650mm depth,
lay the sleepers flat - as they were on the track, not on their side,
lay down 1 side at a time (duh) but going around in a circle, - bottom, left, top, right(dont lay down opposite sides like they used to build log cabins as you will have gaps) so that the ends overlap,
bolt the corners down with 2 diagonal bolts,
keep the length of the pond to the length of the sleeper,
and it wont bow.
so idealy without the side braces you can only go as big as the sleeper size in a square. beyond this and you will need the side bracing poles concreted in.
i keep telling myself, 'maybe this summer hey?' but been saying that for years now so we'll see
We do not inherit the Earth from our Ancestors,
We borrow it from our Children.