Used migration assistant, new imac won't accept user password

I have bought a new imac and on first use, as part of the Set up Assistant process I used the Migration Assistant to transfer my user account from my old imac (OS X 10.6.8) to my new imac (assume High Sierra but can't check).

I followed these steps:

How to move your content to a new Mac - Apple

It took about 20 hours and now the new imac shows that the user account has been transferred but doesn't recognise the user account password (I know the password is correct because it works on the old imac). I can't get past the user log in page and the only other options on the new imac screen are sleep, restart of shutdown. I wonder if this is because of the keyboard setting in the new imac (I didn't get to the stage where I choose language, time etc and the time on the new imac is PDT and I am AEST)? My password is english language letters and numbers so I don't see why this doesn't work on new iMac. Anyway what can I do now as I can use the new iMac at all? Can I undo the Migration? Can I reset the new imac to original settings?

Thanks