Employment Contract, can it be changed after signing just by being handed a letter listing the change?

Hi,
I signed a contract with my current employer after 4 interviews, in each of them i was told my working hours would be 8.30 - 5pm, but that a lot of the time there would be need to stay back (which is fine) On my first day I signed my contact, which listed the hours as 8.30 - 5pm. Then a couple of hours later, my boss came up to me and handed me a letter from a HR guy, informing me that my actual hours are 8.30 - 6pm. I was not required to sign a letter ackowledging / accepting the amendment, just handed this letter. Id this legal? What recourse do I have?
Any advice would be greatly appreciated!
Thanks!

To clarify, I am in Victoria, Australia - Thanks!

According to American law the change isn't valid unless both parties sign it. Since US law is based on English common law, your law may be similar.

Yes this can be done.
But you can deny it and not sign. But if they are not willing to work with you then you can be out.
Even if you sign a contract with them nothing is final. They can make changes and get another signature from you.
Now since they made it an admendment, by you accepting it with no refusal it can go into affect.
If you know an attorney ask them if you can not settle it with the boss. The attorney may charge you a flat rate for his time ($100-$300).

Unless your contract listed a clause that said new items can be changed without your consent, then no, they can't enforce the new time of work day. Now, as another answerer stated, you may be out of a job as well, so if this job means a lot to you, then either go with the new hours, or get a really good lawyer

Once you sign an employment contract and it is accepted by your employer it is binding on both parties. It can be argued that since the contract was presented to you for signature that your employer agreed to the contract as written.

The only way the contract can be changed is by approval of BOTH parties. You should not sign the amended contract and request an appointment with H.R. Present your case to them. I鈥檒l bet they were hoping that you would accept the change without comment, almost validating the change.

If the change of hours is a deal buster for you, and H.R. doesn鈥檛 come through for you, get an employment attorney or whatever they鈥檙e called in Australia.

Good luck!

It's illegal in Australia for a person to work more than 38 hours a week without being paid overtime. Are you intitled to an RDO? This would make up the extra hour a day. I would ring workchoices to find out what to do as John Howard has just changed everything.