I'm Australian and have obtained my Canadian working visa. However rather than flying direct to Canada I am meeting an American friend in LA and driving across the border. I have a return flight booked back to Australia from LA in March of next year too. Will the esta visa will be enough to enter the USA because I have my Canadian working visa and plan to travel there? Or will there be a problem because my return flight is booked later then the 90 days allowed on the visa.
CBP is concerned that visitors to the United States do not intend to live in the US and are not likely to become dependent on public funds. Thus, when you enter, you should be able to show that you are departing the US (for Canada!) and that you have sufficient funds for your road trip.
Be sure to tell the CBP officer that you are driving to Canada and that you need a paper I-94W form. While air travelers' arrivals and departures are recorded electronically, this doesn't happen at land border crossings, thus the paper form is still used when a visitor plans to depart by land. When you arrive in Canada, turn in the paper I-94W to a CBSA officer at the Canadian border; they will return the form to the US as evidence of your departure.
There should be no issue with you returning to the US to catch a flight back to Australia nine or ten months from now. Remember that the US is concerned about people trying to live in the US illegally, not short-term visitors or people transiting to return home. Thus, the so-called 90 day rule, where the clock on a visit to the US doesn't stop if you enter Canada, is intended to prevent visa runs, and it's not a hard and fast rule anyway.