I am an Australian citizen currently staying with friends in America on the VWP and I would like to stay a little longer. This will take me over the 90 days allowed and I'm wondering if I leave the country and go to Canada/cross a border, come back and stay with them for another 5-6 weeks.
No. Generally, going only to Canada, Mexico, or neighboring Caribbean islands and then coming back to the US will not give you a new period of admission on the Visa Waiver Program. You must go beyond those places (e.g. to Central America) to have a chance of getting a new period of admission on VWP when you get back.