Can I enter the UK using my British Citizenship certificate without having a British passport?

Whatever happens at the border, two things are certain:

First, if you enter with your Australian passport, even if you get a stamp granting leave to enter, you do not have to leave within six months, because you are a British citizen. You therefore have right of abode in the UK, and any stamp purporting to place limits on your presence in the UK has no effect. You will not be considered a tourist, as a matter of law, even if the officer somehow thinks that you are a tourist (or any other category of foreigner) and stamps your passport accordingly.

Second, if you have to speak to an immigration officer (which you might not need to if you use the automatic passport gate), the immigration officer has to admit you to the UK if you can show that you are a British citizen. Your citizenship certificate should achieve this.

The specific series of events you will encounter at the border is difficult to predict, but you should have little trouble or delay. If they ask you why you didn't get a British passport, you can explain just as you did in this question.

You should also consider the advice in Traveller's helpful comment: if you are planning to be in Australia for a while, you can apply for a British passport while you are there and use it to return to the UK.