https://www.doc.govt.nz/parks-and-recreation/places-to-go/nelson-tasman/places/canaan-downs-scenic-reserve/things-to-do/tracks/harwoods-hole-track/harwoods-hole-and-starlight-cave-guidelines/
These guidelines indicate the minimal requirements for this trip so it can be done with reasonable safety. They are not a detailed explanation of how to do it.
harwoods holeabel tasmanstarlightcaveguidelines