Since imported high-selenium wheat is an important source of selenium for those living in New Zealand, those who avoid wheat due to eating gluten-free are likely to need other sources of selenium. It may be that beans and rice are imported and supply selenium but without analyzing these foods, we don’t know how much selenium they provide. This is a circumstance where using a supplement that provides the RDA for selenium seems prudent.

Selenium adequacy will depend on individual food choices. For example, vegans in the UK who eat Brazil nuts regularly may have adequate selenium status based on one study that found that eating Brazil nuts can normalize selenium status (https://www.ncbi.nlm.nih.gov/pubmed/18258628) . According to the USDA National Nutrient Data Base, one Brazil nut supplies 95.8 mcg of selenium. The RDA for non-pregnant, non-lactating adults is 55 mcg/day.
Generally speaking, the UK is a low selenium area so crops grown in the UK are likely to be markedly lower in selenium than those grown in the US or other areas with higher soil selenium (https://www.ncbi.nlm.nih.gov/pubmed/22381456). This would affect foods commonly eaten by vegans such as locally-grown grains, fruits, and vegetables. One brief, older (1997) report finds lower selenium status in UK vegans than in UK non-vegetarians (https://www.bmj.com/content/314/7097/1834.2.long).