Can't comment on the Australian tri tip. But 1.5 lbs is about normal for a nicely trimmed tri tip. Untrimmed, they rarely weigh in at more than 2.75 lbs. Whatever you were buying that weighed 5-6 lbs was not a tri tip, unless there were two whole untrimmed roasts in the package.
Modded Brinkmann Gourmet. Basic UDS. Rescued 22.5" OTS. SJS Mini-WSM. Stubbs. B&B Oak Lump.
Cannondale 2009 F8 (for burning off all the great BBQ)