Factor 1: Weather
With it’s Mediterranean climate there is almost no bad time to visit Perth in terms of weather. We experience over 8 hours of sunshine per day, making it one of the sunniest capital cities in all of Australia. Winter never really gets too cold and even in the middle of July you can experience some sunny 23 degree days if you are lucky. There can be heavy downpours through winter so if you will only be here for a short time or plan on doing a lot of outdoor sight-seeing then best to come during a dryer time. The summer months can get quite warm with days over 40 degrees Celsius, so if you are used to a cooler climate then it might be best to avoid January to March as this is when the city experiences the worst of the heat that even locals find a bit too much.

Factor 2: Holidays
Unless you are coming specifically to visit family I would recommend avoiding the major holiday seasons such as Christmas, Easter and school holidays. These are when tourist destinations are at their peak capacity, and prices are at a premium. Upcoming public holidays are listed by the Western Australian Government here, and school holiday dates can be found here. It is worth noting that Western Australia has different school terms to the eastern states, as well as a few different holidays.

On public holidays many shops (including grocery stores) and attractions are shut, or have restricted hours. Cafes and restaurants tend to have a surcharge on food so it can end up being a bit more expensive and difficult to find something to eat. Public transport usually runs on a reduced timetable as well making it harder to get around. In particular, on the Australia Day public holiday the City of Perth runs the sky show which is a major fireworks display on the Swan River. The heart of the city as well as the South Perth foreshore will be very crowded with many road closures for the day which might interfere with your holiday plans (or this may sound like a fun time to visit!)

A benefit to visiting during school holidays are kids carnivals and events that are usually put on, especially right outside the Perth Train Station at Forrest Chase. If you are traveling with children then this might make a good time to visit and enjoy the extra activities.

Factor 3: Nature
Western Australia is best known for being a biodiversity hot-spot, and so one of the main attractions is obviously the beautiful wildflowers. The northern part of the state starts to bloom in June, spreading south with Perth flowers showing up in September. The south-west of the state continues to flower into October and November. The Araluen Tulip Festival in the Perth foothills runs in August or September each year with the exact date decided each winter.

The expansive coastline in Perth also offers a chance to go whale and dolphin watching. Dolphins may be seen during September to May, if you are lucky they may even be spotted off the banks of the Swan River. Whale watching seasons are slightly shorter being either September to December, or March to May, and you typically need to take a dedicated whale-watching tour off the coast. If you are wanting to see the penguins on Penguin Island, ferriess are only available from September to early June.

Factor 4: Events and Festivals
Perth is host to an array of events throughout the year such as the Perth Festival and Fringe World in January and February, Perth Comedy Festival in April and Fremantle Festival in July. Many local councils also run their own festivals or food truck events over the summer months, and there are many markets scattered throughout the year such as the Perth Makers Markets and Scarborough Sunset Markets. These events might attract you to Perth at a certain time of year, or perhaps you are lucky enough to arrange your trip at a time that overlaps with one of these.

Overall
I would suggest October/November as a really great time to head west, as the weather is moderate with most of the rain passing but the super hot days not starting yet, crowds can be low outside of the school holiday dates, and the wildflowers will be in bloom with top attractions like Kings Park looking their best. However if you can’t make it at this time then you will still have a great trip no matter what time of year it is.