Like the flat white, "No worries" really started to take off around the time the work visa requirements for Aussies lightened up. Quid pro quo for Johnnie Howard's support of the Iraq war and all.
Unless you were calling him 'Johnnie' to make him sound small. Which would be a breathtakingly childish and hollow rhetorical device. Stick to the meaningful policy debate.
The actor John Howard doesn't (publicly) get called Johnnie, sure. The politician John Howard was frequently called 'Little Johnnie [Howard]' by detractors, and occasionally this was shortened to just 'Johnnie'; "Did you see what Johnnie did today?"-type stuff. Never used in a respectful context, though.