So, reading between the lines, you want to use post-dated cheques for making payments on a debt? Cheques are a form of promissory note.
Otherwise, it's basically an 'IOU'.
Giving a post-dated cheque is legal under the Cheques Act 1986 (Cth), and a post-dated cheque is not invalid merely because it is post dated.
Whether it's acceptable to the person to whom you owe the money is another question entirely. There's no requirement for them to accept a promissory note, and they're no guarantee of payment.