Iconic has yet to differentiate its product to target certain markets. The petite market is somewhat untapped in the Australian market, unlike the US that has petite ranges in most department stores like Nordstrom.
Points for more petite sizes
1. Ironically the petite market is HUGE. According to the Australian Bureau of Statistics in mid 2010 27% (6 mil) of the Australian population is born overseas, the majority from India, China, Philippines and Vietnam. Average foot size for women in these countries is size 6, average height 5.5 ft. This number is only growing not to mention does NOT include Australian-born people from migrant families (like myself)
2. People from the more extreme upper and lower bound ranges of body sizes (petite and curvy) are more likely to shop online as an alternative to traditional brick and mortar stores in Australia that do not usually cater for them (eg. Country Road, Witchery, Marcs, Sass and Bide, Kookai)
3. Your largest competitors Myer and Davidjones are yet to corner this market. Myer has the smallest (pretty dismal) petite range and DJ only has a curvy range as yet.
4. International students and skilled migrants often have money to spend. They would prefer quality items, (that iconic does offer) relative to the quality of ASOS petite.

I'm not suggesting a stocking a petite range but simply to have more stock of the size 4 and 6 in styles that would suit a petite person. These often sell out first.

If you don't believe me, get a sample size of customers or a focus group and see the responses, I'll guarantee that there is a market of 5 ft 2 and under women like myself dying to spend at the iconic but unable to.