If you still have your original visa in your UK passport and it is still valid then there is technically nothing to stop you using this to leave the country, as several of my family who are citizens have found - HOWEVER - if your visa is likely to expire while you are away then you WILL NOT be allowed back in. This was explained in no uncertain terms to my nephew recently. He found himself about to travel on a visa that had expired two days earlier (he hadn't checked). The security officers at the airport spotted the expiry date as he was about to board his flight to the UK and he was advised that he could leave but would have to apply for either a Resident Return visa or his Australian passport while he was in the UK toget back into the country. As he was about to board his flight, he had little choice but to leave. Fortunately there was a family member still at home in Adelaide. This person had to mail my nephew his Citizenship certificate and birth certificate (copies were NOT acceptable) and he then had to apply to Australia House in London and go through a nerve wracking couple of weeks waiting for an appointment to have his Aussie passport issued. Because it was his first, he had to have the full interview and then travel back two days later to collect his passport (the day he was due to return, fortunately from Heathrow). It was pointed out to him at Australia House that when you get your citizenship the 'recommendation' that you apply immediately for an Australian passport should be interpreted as a 'requirement' if you intend to leave Australia. It also cost him twice as much for the application in London as it would have done if he had applied here.
Really it isn't worth the hassle, if you know you will be travelling to the UK after you become a citizen, then get your Australian passport. Of course if your UK visa/passport is still valid then use it by all means to avoid the queues at the airport - it certainly hasn't stopped my citizen relatives from using their UK passports. Technically you are supposed to travel in and out on the same passport but I know someone who does this all the time and it doesn't seem to cause any problems even though you'd think it would, with the entry and exit stamps in different passports. I'm sure with the technology available that this loophole will be closed before too long and we'll all be waiting in the long queue!