These past days I have been receiving calls from Service NSW asking questions about my business to further my application about the 2021 COVID-19 Business Grant NSW but they keep calling from a private number which I don't get if they are really from Service NSW.
They have a bunch of relevant information and was wondering if it is legit?
That depends on what they're asking. You can get a good idea by the types of questions they're asking and whether the answers/information can be used for scam purposes.
Private numbers are not abnormal from a government department. They don't want people calling back.
Again, it depends on what information they have and what other information they might be trying to get from you.
Alternatively, ask them to give you a number that you can call them back on and check that number before calling back.