Skyrocketing rents have led to growing demand for properties in the city's southeast over the past two years, the Real Estate Institute of Western Australia says.
"They are targeting the cheapest suburbs where they have the highest rents," president Joe White told AAP.
"It's purely driven by return on investment."
Realtor Marcus Blake pivoted his business from predominantly selling to locals to exclusively dealing with interstate buyers because of the surge.
"The demand is through the roof," he said.
"What they're doing is looking at areas that haven't been historically in high demand and that's where they're receiving the huge yields; areas where the sale price is relatively modest but the rental yield and the income are the opposite."
Suburbs 20-40km from the city like Armadale, Thornlie, Brookdale and Haynes are high on the list.
"They're looking at what sold three years ago for $300,000 and now sells for $550,000," Mr Blake said.
"The properties are generally on 600 to 700 square metre blocks and three by ones or four by twos.
"Add another zero on the end if you can subdivide."
Mr Blake said investors were well informed and unbothered by decades-old stigmas around low socio-economic areas.
They're also often prepared to pay more than locals.
"It's an unemotional transaction for these guys from over east, it's just numbers," he said.
"They care about the purchase price, the cost to hold and what they'll rent it for.
"They can spend a little bit more because there's a yield being generated and it's a business decision."
The demand means some buyers' agents are flying to WA from the east every month to search for properties not formally advertised and sold off-market.
"They're the ones that are really killing the pig at the moment," Mr Blake said.
Although there is no hard data about the number of houses being bought up, it's thought to be in the high hundreds per month.
Mr White said the national housing crisis had likely caused the surge, with the shortage of dwellings driving prices and rents up, and in turn attracting investors.
"I hope they're not buying, assuming these things won't change because we're all doing everything we can to ensure the supply of rental properties will increase, which means rents should come down," he said.
"The good side of it is that's topping up our dwindling rental stock."
Mr White said the high demand would inflate prices in Perth's southeast to a point where rental return on investment wasn't as high.
"You'll probably see investors then move on to the next place where they see those opportunities," he said.
There were 379 houses and units sold in Perth in the first week of January, with 2807 listed for sale and 312 available for rent, according to the real estate institute.
The median price in Perth for a three-bedroom dwelling was $525,000 and median rent for the same $580 per week, with 8.2 per cent growth in the past 12 months.
In Armadale, the median price for three bedrooms was $370,000, up more than 20 per cent over the past year and almost 60 per cent over the past five.
The median rent for the same home was $490 per week.
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