70 minutes is plenty of time - if your inbound flight is on time.
Almost all United flights - including ALL domestic flights - arrive into terminal 7 or 8, and all departing United flights leave from those same terminals. These terminals are connected airside, and there is no additional security or outbound immigration you need to pass through. Worst case your two gates will be about a 10 minute walk apart. Boarding for international flights closes 15 minutes before departure, so you've got 40+ minutes from landing which is plenty.
However, personally I would never book this connection. The issues isn't how easy the connection is to make, it's the impact if you don't make it - such as if your inbound flight is delayed. United only runs one flight to Melbourne each day, so if you mis-connect they will book you on the same flight the next day. There's no chance of getting on another airline, as they all leave around the same time so you'll have missed those as well!
If the cause of the delay was their fault (eg, mechanical issue) then they will provide a hotel, but if it was outside of their control (weather, ATC delays, etc) then you (or your travel insurance?) be up for the cost of the hotel.
If you want to risk a 24 hour delay, book this connection. If not, book an earlier flight from Chicago - one with at least 2 hours between flights.