I have a question regarding transit passengers to Bangkok .

A friend and his daughter are flying Bangkok Air from Phnom Penh to Bangkok.

The daughter will then fly on to Australia with Thai Airways about 3 hours later .

My friend will return to Phnom Penh about 5 hours later .

The daughter will have checked in luggage from Phnom Penh .

Do they need to go through immigration for her to collect her bag and then check in on Thai Air ,

or is there a transit area and some way she can get her bag from Bangkok air flight to Thai Air flight ?

depends on the airline. Check with the ailine if they will transfer the bags to Thai airways.

If they don't one will have to go through customs to claim the bags and check in at Thai airways.

Bangkok Airways and Thai will allow this on their Samui route so I imagine its no different. Make the check in at Phnom Penh aware of the onward flight.. Make sure you have the flight number and time of departure handy.

In Bangkok just follow the signs for transiting passengers and there is a desk I believe where you can check in.