I have found it is more convenient to use an electrician during a build - However I buy most of my electrical parts from http://www.sparkydirect.com.au and can save a small fortune - then I have my local electrician install the parts for me.
In the case of lights - you can ask for the builders electrician to install batton holders in between roof battons then install downlight later on and also for power points have the builders electrician install 1 power point on one side of a wall then later on have your local electrician install another one back to back to that one straight behind the original one. This will save you a lot of money. - so the idea is to be strategic with the location of all your points.
Note: In QLD it is costing about $59.00 per Double power point that is not on the original contract.
To install one after building is complete - including service call $145.00