The limits for investment plans are the same as for 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$10,000.
You also can't have more than A$20,000 in total unsettled buy orders at any one time.
If you have total unsettled buy orders equalling A$20,000 or more, your investment plan won't be able to trigger a new buy order on its regular schedule. The scheduled investment won't be processed for that cycle.