Buy orders
There's a minimum trade size of A$10 (including FX and any other fees).
There's a maximum trade size of A$20,000.
You also can't have more than A$30,000 in total unsettled buy orders at any one time.
Example: Let's say you have a A$15,000 investment in Apple waiting on cleared funds and a A$15,000 investment in Tesla in the market, equalling A$30,000 in total unsettled buy orders. Until your total unsettled buy orders are A$30,000 or less, you won't be able to submit further trades.
Sell orders
The minimum sale value is A$5. However, there's no minimum sale value if you're selling your total holdings.
Investment plans
The limits for investment plans are the same as for buy orders, however investment plans may be impacted by the 'unsettled buy orders' threshold. You can learn more here.