Offer accepted..

If my offer on a property has been excepted by the seller and the contract has been signed is there anyway the seller can then accept a higher offer? I've always assumed once the offer has been accepted then that's it but a friend has said the sellers can still accept higher offers?? Can someone please confirm this for me?
Depends on the definition of "accepted".

If the agent has told you the offer has been "accepted", unless pen has hit paper, its not worth anything (including if you've signed anything).

Whenever an agent tells me something like that, my immediate question is "has the vendor signed"

Only once the vendor has signed is it binding. Otherwise, the property can easily be withdrawn/resold to anyone the vendor wishes.

As good measure, I always ask the agent to confirm it with a photo of the signed contract with the phone and mms it to me.

The contract has been signed by both parties and returned to me.

I assume that's it and they can not go for a higher offer?
In QLD if the buyer has the signed contract you have control of the property, if your offer is "accepted" as a verbal or letter of offer (usually a one page non binding promise letter) its still open to being trumped by another contract, however that is very rare.