During the countdown to a jihad-inspired terror attack on the heart of Melbourne, one of the four men behind the plot made what appeared an everyday visit to a suburban hardware store.

But Ahmed Mohamed's arrival at the trade counter of the Bunnings store in Broadmeadows was anything but a pre-Christmas purchase.

CCTV footage released by the Supreme Court on Wednesday shows Mohamed buying 700 nail gun cartridges in red cardboard boxes that are kept behind a locked counter.

The purchase, on December 21, 2016, was unusual enough to arouse the suspicion of two female staffers, who at one point spoke while Mohamed walked away from the counter for one minute.