Birch, who claims to make over $500,000 a year after expenses from the rent he receives from his 200 investment properties, has also obtained mortgage funding from non-mainstream lenders like Permanent Custodians, Perpetual Trustees and BNY Trust Company in the last few years.

He also turned to the same high-interest lender who took him to court last year, Permanent Mortgages, to fund the $1.8 million purchase of a home in Annangrove in Sydney's outer north west that settled in February 2016.

In an emailed response, a spokeswoman for Binvested,com.au, Birch's investing, real estate and finance business, said he had "faced some difficulties last year" but was now in a stronger position after selling some of his properties.

"Having such a large exposure out there in the marketplace, and from speaking to many of the real estate agents that Nathan is in contact with, it is very apparent that the ability of the investor to buy properties has decreased significantly due to the APRA changes," she added.

Melbourne buyer's agent Paul Osborne said investors like Nathan Birch who played the game of "leveraging one property on top of the other are starting to come undone".

"They've borrowed on property which is higher yielding - but which is the first to fall when the market starts to reverse," said Mr Osborne.

Property investment guru Sky News television host Margaret Lomas said Nathan Birch had clearly "just bought and bought based on market sentiment and price.

"He became highly leveraged, which is something we recommend against, and this strategy is not suitable for most people, especially those with personal mortgage, dependents and liabilities," she said.

"He calls himself a 'buy and hold investor' but no buy and hold investor buys property 'like kids buy action figures'," Lomas said.

Walter Nanni, an associate at buyers agents Cohen Handler, said some banks were increasing rates on interest-only loans overnight.

"While this is most clearly seen with specific banks, I'd expect others to follow in due course, contributing to a more challenging landscape for investors," he said.