You have until October 31st to lodge your tax return. I think the 31st July thing was asking for your group certificate, although in my experience the employers dont send it out till mid July anywy.
I did mine online again this year and got back $1160.35 wooo!~!! I would love to use it to go on a holiday to Sydney to see Don's Party, but Im getting my wisdom teeth out under a general on Aug 27th, so thats rather exy, so the tax return is paying for that.
I think next year when I work full time and have lots of deductions and salary sacrifice, I will use a tax agent. Especially whe its all so confusing then.
For my first return i had to pay money back, last year I got about $500 back which went for Melbourne trip, and then this year I got heaps.
If you have to pay back tax, you have until like the following year sometime I think to pay it back, or it might be October...cant remember.