I am from Australia and planning to visit Canada and U.S this year. I got my ESTA approval but not sure about coming to U.S. twice in a month. I will be visiting NY from Toronto and then back to Toronto and then Vancouver. From there I am planning a trip to LV & LA. Will there be an issue as my second visit will be in a couple of weeks time again to U.S. Any help please as I am not sure about this, thanks.
You should not run into any issues. Your reasons for traveling are valid and totally covered under the ESTA program.
I would just take the precaution of bringing along documents that prove your itineraries (your stays in NY, Toronto, Vancouver, LA, and LV) along with a return flight home. This will just show the border guard that you are genuinely traveling and not planning on doing anything illegal during your stay.
Enjoy your trip!