Call your bank. Seriously – just ring them and say you can get a lower rate from someone else. And if they don’t lower your rate, then move your mortgage to that other financial institution. As an example, Ubank have their standard variable rate at 4.09%, with no ongoing fees. If that’s not the interest rate you’re on- call your bank on it.
They may not always be able to match it, but they’ll almost certainly be able to drop it lower than their advertised rate.