'bach' (NZ sense) originates from New Zealand English; one likely source is an informal shortening of the English word 'bachelor' (referring to a 'bachelor's holiday dwelling'), and it may also have been influenced by the Welsh word 'bach' meaning 'small'. 'Bach' (surname) originates from German, specifically the word 'Bach', where it meant 'brook' or 'stream'.

'bach' (NZ) appears to have developed in the late 19th to early 20th century as a colloquial term in New Zealand, probably shortening from 'bachelor' or under the influence of immigrant usages; it became established in New Zealand English to mean a small holiday cottage. 'Bach' (German) comes from Old High German/Middle High German 'bah'/'bach' meaning a stream and continued into modern German as 'Bach', used as a toponymic surname for people living near a brook.

For the NZ word, it initially referred to a simple dwelling used by bachelors or a small building ('small' sense influenced usage) and evolved into the general New Zealand term for a modest holiday cottage. For the surname, it originally meant 'brook' and has remained a family name derived from that geographic term.