The way HECS is indexed is based on inflation rates. They announce the amount it will be indexed in early May (though you’ll be able to find out pretty accurate estimates as far back as March), and then apply it on 1st June. As long as you make whatever payment you want to make before 1st June (try to aim for no later than about 20th–22nd May, to make sure the transaction goes through in time) it makes literally no difference when you do it. Pay extra today or pay it on 15th May 2024, it’ll have the same effect. Except that if you pay it today, you’ve lost that money and it’s no longer earning interest for you in the meantime.