When and how much are you fined for not completing your tax return?

Excellent question! If the IRS or state owes you money, then when you finally do your tax return, they will not fine you. In fact, they are happy you didn't file on time because they had a chance to keep your money for a while and make interest on it. They will probably pay you some interest, but believe it or not, they will make you claim that interest on your taxes in the year it is paid. you will get a 1099-INT.

If you end up owing them money, they will make you pay interest / penalty on the amount due from the day the amount should have been paid. According to the IRS,

"The penalty for filing late is generally 5 percent per month, or part of a month, up to 25 percent of the amount of the tax shown due on the return. The penalty for paying late is 1/2 of 1 percent per month, up to 25 percent of the unpaid amount due.

The IRS recognizes many people drop out of the system because of personal problems, including serious illness, a death in the family, or loss of financial records in a natural disaster. Depending on the situation, informing the IRS why returns have not been filed could result in a waiver of penalties."

So, filing on time but not paying is much better than not filing at all...but if you are getting a refund, by all means, file as soon as you can!

(after I wrote this, I realized you are from Australia...the tax laws there could be different than where I'm from, the USA)