You should always assume formal unless you have been advised otherwise. It's generally ok to end up a little overdressed, but it's not ok to end up underdressed.
However, software companies can vary. It is going to depend greatly on the culture of the company.
If you can, do some research on the culture before the interview. Is there a "team" page on their website? How are the employees dressed? Is there a certain larrikin'ism evident in any of them? Or, can you find any team shots of them at work or play on the company's social media sites?
This will give you some sort of insight into what it's like to work there, how they dress, and accordingly how you might be able to dress for the interview. I must stress: don't wear a t-shirt just because one person has in a photo; you want to always lean to the safe side.
As an anecdote, I once went for an interview where I wore a collared shirt and dress shoes - but with jeans (nice ones!) It's kinda like the hipster way of dressing nicely. I only did it because I knew the company was laid back, and the boss actually commented that he appreciated I wasn't in a suit. Suits can portray an air of unfamiliarity, potential cultural mismatch and a lack of research - if the company culture is a laid back one.