When buying a security for the first time, the ASX prohibits the settlement of any transaction that will establish a holding of less than $500.00 based on the closing price of that stock (also known as an 'un-marketable parcel').
As such, for your first holding in a security you cannot submit an order valued at less than $500.00 (excluding brokerage). Additionally, if using the 'market then limit' price type an additional price buffer will be added, which varies based on the pricing of the security (orders using the 'Limit' price type are not affected by the buffer requirement).
If your order does not comply with the minimum parcel and buffer rules, you will be shown the below prompt when attempting to submit your order to market:
Due to these requirements, we recommend that your cash account has a minimum balance of $550.00 before establishing a new holding on the ASX.