Thanks gents. I had a feeling it was a native from its appearance but couldn't find anything like it in the CNM "Other Wildflowers and Herbs" field guide. Still can't, in fact, because none of the other images for Epilobium billardiereanum show much of the foliage, and it doesn't look like any of the photos in Don and Betty's book either. But I agree that it matches pretty well with Plantnet and SE-NSW lucidcentral descriptions.
Can I suggest that one of my images (which for once is pretty sharp and clear) be added to the field guide to help others who just have foliage to go on? Although perhaps wait until it stalks and flowers (I'll be keeping an eye on it) when we may be able to identify the subspecies.