The OP is in New Zealand. I'm in Australia. Our countries are more similar than either of us would like to admit.
Here we don't have fixed interest loans for the duration of the loan. You can fix at the start of the loan, but usually only up to three or five years (strictly speaking, you can go longer, but you often pay a premium that makes it unaffordable). So most people have variable home loans.
However, we do have a magical beast called an 'offset account'. An offset account is a transaction account. It pays no interest, but the balance of the offset account offsets your variable home loan balance when calculating the interest payable on your home loan. Easiest way to understand it is by example.
Let's say I have $500,000 outstanding on my home loan. I also have $300,000 in my offset account. The interest I pay is calculated on a net balance of $200,000, not the $500,000 outstanding.
Offsets are great if you're disciplined with your money, since money into the offset gives an effective return of whatever your home loan interest rate is after tax. You put all your income into your offset and withdraw your expenses as you accrue them and that helps save interest day to day.
But they can be particularly effective for FIREes. Particularly someone like the OP who is obviously a high income earner. As a high income earner, he will have the ability to borrow lots. But he also has a lot of cash and doesn't really need to borrow. Given he intends to FIRE in the short to medium term, his ability to borrow in the future will be less than his ability to borrow now. But he can get a loan now, using his high income borrowing capacity and 'reserve' it for when he's FIREd.
Let's say our OP borrows $1.5m to buy a house in Auckland. He doesn't really need to borrow, he could pay cash. But he borrows that money and uses $1.5m of his cash to fully offset his home loan. He then pays no interest on the loan (he will have to make a minimum payment, but it will all be principal, and it can be set up to draw directly from the offset).
The offset account becomes his emergency fund. It also becomes a quasi-bond allowance. When it's cost effective to do so, you sell down equities (or use distributions from equities). When it's not, you take some cash from the offset. You may then have to pay some interest since your net home loan balance is now positive, but when it becomes cost effective to do so again, you draw from equities to fully replenish the offset.
I'm not sure about the OP's situation as a NZer, but in Australia, if your home loan with offset is secured against an investment property, rather than you PPOR (Principal Place of Residence), any interest you pay in that scenario may also be tax deductible.