Hello Brendan!
The tab content is visible using IE 11 on other sites.
I can see on your site this CSS code, that is hide the tab content:
/*media all*/
.mk-tabs-pane {
padding: 0;
display: none;
}
This css is added by Jupiter theme, according to the URL of the CSS:
http://www.maser.co.nz/wp-content/themes/jupiter/assets/stylesheet/min/components-full.css,qver=5.1.7.pagespeed.ce.kJnojOHbIX.css
So I propose to ask the developer of Jupiter theme about it.
I hope this information is help to solve the problem.
Peter Rath
WebshopLogic