On separate tickets I would allow a 3-4 hour transfer time in Sydney. United land at a very busy time in SYD.
Melbourne is slightly different as the domestic and international terminals are in the same building. But you still have collect bags, check in, go through emigration and security. But 3 hours is probably more than enough.
Be warned that domestic flight baggage and cabin baggage allowances on QF and VA are a lot less than you are probably used to and are being strictly enforced so carry on only might be a problem.
Also don’t forget you will need visas. Use only the Aus gov web site. Don’t fall for third party service providers.
https://immi.homeaffairs.gov.au/visa...sa/visa-finder
If you can avoid Jetstar (QF owned LCC) and Tiger (VA owned LCC) for domestic flights.