Answer:
There are some lotteries in Australia that use a 6/45 matrix, however there is no such lottery called "Australia 645 Lottery".
If you've received an email, letter or call about winning a lottery that you did not enter, it is a scam. The scammers will try to get you to reveal personal information, or to send them money in order to "claim your prize". Do not respond to them, don't reveal anything, don't deposit any cheques they send you, and don't send them any money under any circumstances.
Sorry - we all dream of winning the lottery, but in this case it is a scam.