I am Australian and planning to travel to the US in November for a month. I will be flying into the US and as part of my trip then driving to Mexico for a week and then back to the US. Can I get a visa waiver for this or because I will be entering from Mexico 'by land' do I need some other sort of visa?
In general the requirements for entering the US by land are different than when entering by air. An ESTA is not required, but a completed I94W form is (available at the border).
However in your case as you will be entering the US by air first, you will be given a "visa waiver" stamp in your passport on the initial entry, which will be good for up to 90 days.
For the most part, this stamp will become invalid when you leave the US, however there are exceptions for travel to Canada and Mexico (and a few of the Caribbean islands), such that if you visit those countries then your Visa Waiver stamp remains valid, and thus when you re-enter the US from those countries there is no need to go through any additional formalities.
Specifically in your case, on re-entry to the US from Mexico you will still need to present your passport, which they will scan and confirm that you already have a current visa waiver entry (what used to be a physical "I-94W" card, however they did away with the paper cards several years ago) and they will allow you back into the US without re-stamping your passport, and thus without changing the date on your VWP stamp.
Your visa waiver that you used to enter the US initially should be fine for returning from Mexico. If your whole trip is only one month long, then you won't need to worry about the fact that the 90 day maximum clock does not stop when you enter Mexico.