Am a bit late to this thread but having had a skim through the comments most of them are laughably misguided in terms of the legal rights of the professional photographer over the OP.
Under Australian copyright law, the OP gained sole copyright over the photos that he took as soon as he pressed the shutter. That gives him unfettered rights to commercially exploit his photos.
The only way that the professional could restrict those rights is by virtue of a contract containing explicit terms prohibiting him from using/selling the photos. There is no way that the a court would read in such an onerous term into the very loose arrangement described here. I very much doubt that there is any contract between the OP and the pro photographer governing the shadowing arrangement, but there clearly is no term covering assignment of copyright or prohibition on exploitation of photos.
The one legal claim to the photos of the OP would be from the part of the bride. If we changed the facts a bit here and the OP wanted to sell his photos to a bridal magazine, the bride may be able to restrain this by bringing an action for breach of confidence. However, even this would be a pretty weak action given the reluctance of Australian courts to recognise any tort of privacy. Her only strong action would be against the professional (who she has a contract with) for his negligence in allowing the OP to tag along without requiring him to enter into a contract to restrain his use of the photos. But I digress.
The only issue at stake here is the OP's ethics. And personally I think the professional is the one who should be grateful that the bride isn't tempted to take him to VCAT for stuffing up the coverage: http://newsfeed.time.com/2013/01/13/wedding-photographer-sued-for-missing-the-kiss/
Think a lot of the comments here are fuelled by "professionals" feeling a bit insecure about second shooters
[FWIW I'm in my final year of a law degree]
Once you've got a few years' IP law under your belt, you might see things differently. I'd argue that the OP took the images as an agent of the pro photographer (as he would have been under the supervision and guidance of the pro and his attendance at the event would have been under the pro's direction), and consequently the pro owns the copyright (subject to the contract with the bride and groom....as they are the client, depending on the wording of the contract, you might find that they already own the copyright - take a look at s35(5) of the Copyright Act.)
Given the circumstances, I have a hard time seeing how the OP could own the copyright. I suspect he'd argue that as the person pressing the shutter button, he was the creator and therefore the owner. But given that this would have been a private event, with the OP attending under the direction of the pro photographer, if this ever became a serious issue, I'd suggest the pro (or the bride and groom) would have the winning argument. (Law degree and 20+ years experience.)
Back to the OP, I'm happy that you discussed it with the pro (even if it wasn't 100% your desired outcome). We'd have 2 or 3 people come and do work experience with us from high schools and universities each year. And while we're not in a photography related industry, everyone that works with us always acts on their best behaviour and we're delighted to have them, even though a certain percentage will ultimately become direct competitors. But we like doing it. Not only do we identify potential employees, but it is always useful having contacts in other firms or different specialities. Hopefully, you've kept the relationship with the pro on a good footing as you never know when your paths might cross again.