I lived there for a while, bought a house in Coogee just south of Fremantle. To be honest I was disappointed with the local fishing and found myself regularly heading north to Kalbari, Shark bay or Exmouth etc for arm pulling species.
Most people around the Fremantle area targeted herring, which aren't really like herring at all, more like a small Aus salmon or roughy. These can be cast back out in a few areas in the hope of something bigger but the bigger species get a fair amount of pressure from anglers who put many hours in targeting such things as mulloway etc and understand their feeding patterns.
Anywhere that has either natural or manmade structure will often hold many schools of the small herring and often squid which on light tackle can be fun. Also worth putting either out as a hole bait just incase.
South of Perth you may find some very large schools of Aus salmon which are very good sports fish but not so good to eat, most are commercially targeted for the pet food industry or trap bait. However if bled and prepared properly, salmon can be ok to eat if you keep smaller ones under 3kg. These will take just about any lure or bait you can get to them.
Unless your wife uncle is a pretty hardcore angler with a boat capable of heading far offshore don't expect to much from local inshore waters. I was with marine rescue in that area for a couple of years so got to see and experience for myself how hard the fish can be to find around there. Some boat ramp areas will see 2000 boats per day on a good weather weekend, which accounts for a fair amount of pressure on some species. I also spear fished around that area, so got to see what species were on offer, or should I say lack of!!!!
I moved from WA back to NSW as the crime rate was pretty poor, leaving a car in a carpark behind sand dunes after dark to fish the beach was a common way to get you window smashed and everything stolen from the vehicle. There are lots of Poms living around the Perth area, you will think you are back in the UK!!!!!
The best advice would be to search out local fishing forums in the area and introduce yourself as many Aussie anglers will often accomodate you in showing what's on offer locally, more than they will to other local anglers obviously. Also be prepared that it can be pretty windy so if you aren't overly seasoned to rough water in boats take some seasick pills ahead of heading out. Also be aware great whites will come very close to shore, I saw several while living over that side. Water temps always seemed cold to me, I always dived in a 5mm suit which out of the water in the air temps of mid to high 30's was a killer!!!!!
When ever we had visitors from the UK we drove them up to Monkey Mia to see the dolphins (one day drive). The fishing up around there was good especially after dark if you fished from the shore into any deeper looking water, with various big fish and numerous shark species etc. Kalbari beaches after dark were extreme, have plenty of hooks and heavy line!!!
A really worth while book to get is the fish finder magazine which gives very good details on what and where to fish which represent excellent value in turning a fish-less day into a good one, knowledge that would take many years to gain North Australian FISH FINDER – Australia's most detailed fishing map book
dont be misled by the cover, it doesn't just cover the NT but all of Aus, maybe the wife uncle already has one.