As a background story, I am part of the IT personnel at the company.
The company wants to create a system to automate the invoicing and billing process using our internal system, because at the moment we're doing all of those manually by looking at the transaction history on our system and then copy/paste to an online accounting system service.
Obviously I need access to be able to request an API call to that accounting system online to create the invoice/billing from our internal system. Additionally I need full access to the system to do that (the accounting system we're using is not very good to be able to implement a minimum access policy for this, but it is good enough for us so far).
Consequently since we're using the same system and the same account both for our customer and internal accounting stuff (staff paycheck, bills, etc.), I could see literally everything (we're a small company with <20 staff in the office).
Is it ethically wrong to have a peek at staff paychecks?