You will run into these occasionally in Nelson. They are the larvae of the spitfire saw-fly. The larvae can be quite big and fat, say up to 8cm long and will eventually turn into a quite small wasp-like fly. The ones in this photo might be the steel blue sawfly (Perga dorsalis) which is fairly common.
The Spitfire’s nickname comes from a smelly, yellowy goo they dribble when frightened. They feed on gumtree leaves at night then cluster up during the day for protection.
The link below has more information about them.
https://www.environment.sa.gov.au › goodliving › posts › 2018 › 01 › spitfires
Nelson Coastcare 