Night caller in Brisbane

I've recently moved to Brisbane and am woken each morning (usually at 4.30am) by a simple call that i need help to identify.

It sounds like... 'do doo' ........'do doo'.............The  second 'doo' is lower in tone.

It sounds very calm.

it's a clean crisp noise, not a furred blurry noise like doves.

It's repeated - usually at regular intervals but not always. 

i don't hear it during the day .

I've tried looking and listening on youtube and birding sites but no luck so far.

thanks for your help in identifying this.

We've named it 'the 4.30 bird'!

sian