For Keyword [website design perth] Google adwords set for Australian English (if it makes a difference) says there were 720 searches per month on average.
For Keyword [web design perth] there were 2,900 searches.
Not only that, but Google retrieves 787,000 pages for [Website design perth] (many pages, few searches) versus 540,000 for [Web design perth].
Read about Keyword Niche here: Niche
Obviously, more searches and less pages is better.

Catch - what I am getting for [Web design Perth] from Google is a list of local businesses at the top of the page - 10 links with a map of Perth. That comes above all the other listings. You get yourself listed in that for free here:
http://www.google.com/local/add/busi...gl=AU&hl=en-GB

It is probable that that local listing is getting most of the traffic. Advice - Optimize for Web Design Perth. Create a file name called Web_design_perth.htm and appropriate text and then link to it from a lot of pages with keywords or titles "Web Design Perth." It should have about 60-100K of text - the more the better, and get yourself listed in the local business center.

SEO - Search Engine Optimization