Mikebabz food truck at ANU Popup Village - I've had their HSPs 3 times already and they are pretty good! A small one is $10 but it's actually quite filling! They also come with cheese on theirs (some places I've been to don't), and the workers are super nice too! You can also choose between beef, chicken or mixed for the meat
Patissez - they have one in Canberra/near city and one in the south somewhere. They're known for their freakshakes:

but other than that they also are a nice cafe and stuff. A bit expensive, but their freakshakes are so good + quite a few diff flavours (I got a chocolate cookie flavour one last time iirc, which is 2nd of the right)! They also don't look that big when you buy it but it's actually so filling! It's $13.90 for the normal size I believe.
If you just go for a freakshake that's already enough