Polling like this is done over the phone, they take numbers at random, and when you start getting into the 1000s in terms of a response, the samples become pretty accurate in terms of representation

https://en.wikipedia.org/wiki/Sampling_(statistics) theres probably more info here

also the snappy headline saya 91% sure, but looking at the article

86% agree or strongly agree that government and opposition should not politicise trans issues for political gain  89% agree or strongly agree that LGBT people deserve to live with dignity and respect  91% agree or strongly agree that trans people should have the freedom and choice to live their lives in the way that makes them happy  81% agree of strongly agree that trans people deserve the same rights and protections as other Australians 

for reference the population was 26million at the last census, and 91% of that is something like 23,660,000, so there's still a few million people who'd love to attack queer people, i've met plenty, but honestly most people dngaf about trans people, and have not met one irl