Common along roadsides with other wild Brassica. These plants with swollen rootstocks - such plants have been called Brassica napus var. napobrassica in New Zealand by Heenan et al. (2004) N.Z.J.Bot. 42: 815–832 and also B. napus subsp. rapifera by the New Zealand Plant Names Database - see https://nzflora.landcareresearch.co.nz/default.aspx?selected=NameDetails&TabNum=0&NameId=CF5A2387-29A5-4393-9032-1F4E02342C43.

I presume the name B. napus subsp. rapifera is correct..