Flight to Sydney from UK with breaks

I intend to fly from the London area of the UK to Sydney to arrive in
Sydney on 5th April and leave Sydney for the UK on 30th April. I would
like to break the journey in each direction for a night or two. I intend
to fly from Sydney to Cairns or Townsville on the 10th, take the train
between these two cities and return to Sydney from the other on the
20th.

I would like suggestions on the best websites to look at for websites
for my flights. And any other thoughts you may have.

--
Andrew Nightingale of Cambridge (UK)