It might be... the trouble is, there's a lot of similar plants available in Australia, often sold under the name "Echinopsis seminudus" that look a lot like your specimen... I know, I've got a few of them. From what I've been able to discern, they are mostly hybrids, and it's theorised that they're a mix of subdenudata, eyriesii and oxygona. The flower varies between pure white (possibly the original "pure" subdenudata clone) to pale pink and lemon yellow.