BPAY merchant accounts have a minimum and maximum limit set for each incoming payment (the Westpac and Commonwealth Bank FAQs refer to these limits, for example). This is a security measure put in place help protect both merchants and consumers.
If you are unsure what these limits are set to for your BPAY account, please contact your bank to find out.
If your investors need to pay you more than what your BPAY maximum limit allows, don't worry, they can still pay you via BPAY if they want. They will just need to make multiple payments, each of which is for an amount below or equal to the maximum limit, in order to pay the full amount. In this scenario, the instructions on their application invoice will explain to them how to do this.