Gunnera is a prolific plant which can release 1000s of seeds each year, however it can be removed physically or treated with herbicide.
Physical removal entails the excavation and removal of the root system and any seeds which have been dispersed into surrounding soils. Without the root the plant will not regrow and the removal of the seed infested soil will prevent any new plants growing, giving an immediate solution. The excavated material is then disposed of at a licensed landfill.
Chemical treatment only affects germinated plants and not the seeds which are present within soil. Eradication takes 2-3 years which is made up of two treatments each year during the summer (July-September). Treatment needs to be persevered with to achieve eradication as if a single year of treatment does not take place you may be taken back to step one with new seeds germinating in following years.