I would insist on your LTV being based on what you need not what you have approval for.
From Nib’s website the underlining is mine:
Did you know you can use the equity you've built up in your home to reduce the interest rate you pay on your mortgage? That's the beauty of our LTV Mortgage. In a nutshell, we calculate the amount you still owe on your home against the current market value of your home. This gives us your LTV ratio (Loan-To-Value). The lower your LTV ratio, the lower your interest rate and the greater your savings.
[broken link removed]
This article might also help your case:
[broken link removed]