There's a paved area in our back yard that ends up sitting under a couple of cm of water for about a week after every period of heavy rainfall, and some of the cement pavers are badly cracked and need replacing. So rather than spend money on installing grated drains and re-paving a large area, I decided to just rearrange the existing pavers and use the space left by the broken pavers to install a Koi pond (about 3m x 1m x 35cm deep). The bedrock is fairly flat and about 35cm below ground level, so I'm simply digging out the soil and will cement in some 20x20 cm pavers ($1.60 each) (standing upright) to form the edges of the pond. A black plastic 6m x 4m pond liner ($169) should stop any groundwater seeping into the pond, and I'll install some ag pipe and gravel along the back of the pond to direct the run-off into a nearby existing drain ( although the lack of any significant slope will make this 'interesting' to get right).
Conveniently there's a wholesale Koi 'fish farm' only a few km from our house, so we visited there yesterday to see what's available. DW and the kids were very impressed by the large, colourful Koi (some costing $350 or $475 each!). Fortunately the fish farm also sell small, young Koi for only $6 each (or 10 for $50), which is what I'll buy when our pond in finished. Apparently it will only take 5-6 years for these to grow to the size of the $300+ Koi (assuming they don't die or getting eaten by birds or cats!), although I doubt they'll be as colourful or valuable as the 'show' Koi on display. I don't want to spend too much money on this 'project', so instead of spending around $1,000 on an external pump and separate filter with UV treatment, I'll just buy a cheap ($120) submersible pump/fountain with a replaceable sponge filter bloke ($6). From past experience saving money on the pump will mean having to be very careful not to overfeed the fish, and having the siphon some waste off the bottom of the pond every few months.