floydie: running power supplies at very low load is flakey. you get all sorts of harmonics int the supply that seems to get dampened at closer to spec loads.
Power supplies are tested for EMC at full load and at no load. Power supplies sold in NZ must comply with these requirements, and you are entitled to ask the supplier for a copy of the compliance certificate (they must have it on file, can't just refer you to the manufacturer) but I suspect Noel Leeming etc. will just stare at you. Rubbish that don't comply can be flaky and not legal, but it is surprising how much of it is in the market.