It is an insurance nightmare for the Canadian rental company if a driver has an accident in the US.

We are Australians who want to take a rental car from Vancouver to Alaska and back through the Yukon to BC, Alberta etc back to Vancouver. We can't believe that there seems to be a problem with this for Canadian rental car companies. Are there any car rental companies which would allow us to do our trip?
It is an insurance nightmare for the Canadian rental company if a driver has an accident in the US. Add in your Australian drivers travel adventure companies license which might not be acceptable to the rental agency and you might be stuck riding the train or the ferry.
Many thanks for your comments. We are now looking at renting from Bellingham, USA, and having an 8 weeks rental. Many do not like renting longer than 8 weeks but that should be enough. We are hoping to do day boat tours within the Inside Passage as we go. eg. from Valdez. The Alaska Marine Ferry would have been great but a bit late to do all? We understand that the Inside Passage is pretty wet during August so it may be quite misty?
Also, let me state that, technically, the rental car companies do not authorize driving their cars on unpaved roads. The Alaska Highway, during my 2400-mile trip in each direction, may have had approximately 30 miles of unpaved surface. (this was in early April, and you can hope conditions are better later in the season, after yearly road repairs)
Thanks everyone. I just booked a rental car from Bellingham from one of the big 4 international franchises thinking that I could take a car from the USA through travel adventure companies Canada to the USA (Alaska) to get over the Canada problem from doing so. After taking my booking I asked that the car be a 4 door one etc as I was going to Alaska. The reply was our cars are not allowed into Alaska. So I will try ringing them direct to see whether any of them (Canada or from the USA) will let their rental cars into Alaska. Never struck this problem in Europe travel adventure companies or anywhere else? It is like you can rent their cars as long as you just poke about town !!
Well alls sweet. The large franchisee travel adventure companies can supply a car to use in USA, Canada and Alaska. Good to see a sensible outcome. I am sure that we will enjoy the scenery as well as the friendliness and hospitality of the North American people.