Hope this helps:

Virgin
Many women fly while pregnant and have no difficulties. We recommend that you discuss your travel plans with your doctor before flying, particularly during the later stages of pregnancy.

We require you to travel with medical clearance during pregnancy if the following applies:

* Pregnancy with complications.
* Multiple pregnancies after 34 weeks.
* Normal pregnancy after 36 weeks
Jetstar-
If you are pregnant and your travel is Domestic Travel:

* you have a normal pregnancy , no medical clearance is required.
* you have a complicated pregnancy , medical clearance is required.
* you have an uncomplicated multiple pregnancy , medical clearance is not required provided travel is completed by the end of the 36 th week of pregnancy. There must be 28 Days between the date travel is completed and the date on which you are due to give birth. You must carry a letter from a registered/licensed medical practitioner stating that your pregnancy is progressing normally and specifying the date on which you expect to give birth.

If your travel is International Carriage:

* you have a normal pregnancy, all of your travel must be completed by the end of the 36th week of your pregnancy. There must be 28 Days between the date travel is completed and the date on which you are due to give birth. You must also carry a letter from a registered/licensed medical practitioner stating that your pregnancy is progressing normally, and specifying the date on which you expect to give birth.
* you have a complicated pregnancy, medical clearance is required.
* you have an uncomplicated multiple pregnancy, medical clearance is not required provided travel is completed by the end of the 32nd week of pregnancy. There must be 56 Days between the date your travel is completed and the date on which you are due to give birth. You must carry a letter from a registered/licensed medical practitioner stating that your pregnancy is progressing normally, and specifying the date on which you are due to give birth.

We do not represent that travel is safe for you at any particular point during your pregnancy. You must seek advice from your own medical practitioner. The periods referred to are only our minimum requirements
Qantas-
Medical clearance is only required if you are having complications of pregnancy. International travel is not permitted after the 36th week for routine pregnancies or the 32nd week for routine multiple pregnancies. Some countries place limitations on the entry of non-national pregnant women. It is best to check with the local embassy or consulate if in any doubt.

For domestic travel, there is no restriction for a normal uncomplicated pregnancy but medical clearance is required if you wish to travel past the 36th week for uncomplicated multiple pregnancy.