Vogue has also warmed to the idea of using fuller figured women, recently putting the glamorous and curvy Kate Upton on its American cover.
Designers here are also changing their attitudes towards what measurements models need to shrink down to and a recent international study - commissioned by So Fabulous for Littlewoods.com that specifically designs for women of sizes 14-32, found size 16 women are the happiest and most comfortable in their own skin.
Ms Williamson, 23, was talent spotted by her current agent, Amanda Bransgrove of Monarch Models, at an event in 2011.
"It was a bit surreal because you only see things like this when you're reading a magazine about a famous model who was approached on the street.
"It was surreal, then it was funny and then it was a reality."
Within months, Ms Williamson was booking photoshoots, campaigns and runway jobs. But being a plus size model isn't a full-time career option here, so she continued studying English literature at the University of Auckland and working part time. These days, she has a major job - such as a six-page spread for plus size bridal wear or a runway walk - about every three months with bits of work in between.
In her opinion, plus size modelling was more specific in shape than those size 8 or smaller.
Different designers want different figures and so Ms Williamson had to go up and down sizes to fit contrasting ideals on what is the ideal curvy body. While she stayed around a size 12 or 14, some jobs required her to be a bit smaller or larger.
"Everyone wants the beautiful shoulders, the large chest, hips round and curved and a comparatively small waist. That's often much more unrealistic than rake-thin," she said.