Not sure about NSW but in WA this is what I do.
1. The lease survives the sale.
2. The bond is the tenant's money and should be held in trust somewhere, either with an agent, an owner or a government department.
3. Complete a bond transfer form noting your new ownership.
4. The agent or owner should issue you a cheque for the bond ,which you then lodge at the appropriate place. If it is with the government, the change is simply noted and you get a record of it.
5. Issue the tenants an invoice for the bond top up with the appropriate notice period to pay (60 days in WA).
6. Do a Bond Variation when you receive the top up, adding it to the existing bond.