Domestic to International Transfer at Sydney

From Virgin Australia's Connecting Flights page,

Domestic Check-in Connecting through Sydney

When you check in at a domestic port, your bags will be checked through to your final destination. You will receive a transfer wallet, which contains a map of the transfer port and a bus pass to use in Sydney. Upon arriving in Sydney, make your way to the transfer lounge at Gate 46 and check in for your international flight. Once you have received a boarding pass for your international flight, you will be shown downstairs to the transfer bus – which will take you to the International Terminal. If you are issued with a boarding pass at the transfer lounge, you will not be required to check in again at the International Terminal.

Sydney Airport, as noted, has its domestic and international terminals quite a distance apart. As such you'll need to get a bus or train between them.

Your best bet is to check with the Virgin transfer desk upon arrival and explain your situation. If all goes well, you'll be able to use their courtesy coach. However their website claims it's for use of VA customers and its partner airlines only, of which United is not one. Australians aren't necessarily sticklers for policy though, so there's a fair chance you'll be able to use the service.

If not, your options are public transport, both of which cost between about $5.50 and $6.00 Australian:

• The T-Bus leaves from outside the arrivals and baggage collection area of the Virgin terminal. The bus stop is in between the taxi ranks and the main road, across a pedestrian crossing. Traffic can be bad, depending upon when you're passing through.
• A train line passes under the two terminals and only takes a couple of minutes. These come about every 15 minutes or quicker, but are down several flights of escalators or lifts. The way to the train station is very well-signed.

To add to other answers - you can also get a taxi between the two terminals BUT the taxi drivers on the 'taxi rank' may be unwilling to make the short lowish-price (not low) trip between terminals as they lose their place in the queue. So standing at the main entrance and acquiring a taxi from an arriving passenger is probably easiest.

Overall, Sydney airports (both ends) are very accustomed to transferring people between terminals and you should have no problem getting there promptly.

However, as you are probably aware, the normal check-in time for an international flight is 3 hours, you usually have no problems at 2 hours preflight and somewhere after that things start getting marginal. You are in the marginal zone, as you know. In your position I would check to find out how Gregs advice " ... Upon arriving in Sydney, make your way to the transfer lounge at Gate 46 and check in for your international flight. ..." applies to your flights. ie is "Gate 46" a universal transfer gate - if so all is well, or if not, what is the equivalent gate for your flight - it is almost certain that this arrangement applies for all airlines flying internationally out of Sydney.

As you are single ticketed with UNITED, that seems the logical place to start asking.