Issue with Appointments not sending reminder emails at correct times

I have a multisite install and use this plugin on all of the sub-sites in my multisite. One of the sub-sites is http://dimension3architects.archwebsite.com. They are based in Australia on Brisbane time. This timezone is set in the general settings but it doesn't seem to be corresponding with the plugin settings. There are reminders set to be sent out at 24hrs before an appointment and another one at 1 hour before the appointment.

This is not happening when we test it. A reminder for a test appointment went out 3 hours before the appointment time not 24. I think the reminders are being sent out according to the server timezone instead of the timezone set for the individual sub-sites.

How do I get the appointment reminders to send according to the settings for the sub-site, in this case it would be for Brisbane time, but we have sub-sites for people in various timezones where this can present an issue.