I refinanced the loan to a major bank as a $1,540,000 investment loan with interest only repayments, over a new 30 year term.
The interest only repayments are what lowered their repayments straight away. The 30 year term matters later. An interest only period doesn't last forever. When it ends, the loan reverts to principal and interest over whatever is left of the term. Starting a new 30 year term means the principal is repaid over more years than if the loan had stayed on what remained of the old one.
The bank assesses an interest only loan on the principal and interest repayment that follows, so the application had to stand up on that basis. One borrower is employed on a strong income. The other is self-employed, and the bank's policy let us use the most recent tax return on its own, which was the higher year. The property also has a granny flat, and rent from both the main house and the granny flat was counted. At 67% LVR, there was no lenders mortgage insurance to pay.