OK guys, hopefully MArio can shed some light!

I am dual citizen UK/AUS my wife is Thai, my son can get UK/AUS/Thai passports - ok legally he cannot have all 3, this I know. For travelling it seems the best thing for the baby is to travel to the UK on a UK passport & to Aus on Aussie passport. My question is, will he need to have a visa to re-enter Thailand if he is to travel on one of his non Thai passports? our first trip he will be about 6-7 months old, incase it matters

I am not clear why your child can't have 3 passports, all 3 countries allow multiple nationality. Despite rumours to the contrary, Thailand certainly allows it in your child case.

Yes, if your child enters Thailand on a foreign paspsort, the child will be subject to immirgaiton rules and needs a visa and extensions of stay. Therefor apply for a Thai passport and leave and re-enter Thailand on that one. While checking-in at the airline you show both passport to confirm no visa is required and at immirgation only the appropreite passport.

When travelling or the first time with the child Thai immirgaiton advises to take a copy of the birth certificate with you.

AH great thanks very much, my issue is not wanting to have to apply for another visa for the UK/AUS as this is just a headache & costly. SO in theory I should just be able to turn up to Sydney with his Aussis passport & enter as a citizen, even though he will have travelled on his Thai ?

Yes, that is done all the time. Airlines and immigration are very familiar with this situation.

You can join your family in the lines for Thai passport holders at immigraiton.

Thanks again, you are a star!