When host Scotty Cam arrives on The Block site and whistles to get everyone's attention, it's usually to announce the next challenge.
That's when the contestants take a break from building, have some fun, and maybe even win a bit of much-needed cash for their building budgets.
But when he calls together the Blockheads this week, it's for a very different reason. With the COVID-19 global pandemic fast approaching, it's time to call "tools down", possibly forever.
"I'll always remember looking up and seeing all the people in hi-viz hanging off the scaffolding all the way up to the roofs of the houses," Scott, 57, tells TV WEEK.
"There were about 150 people – contestants, crew, everyone – and me giving the speech, 'London's gone into lockdown, without notice…'
"Then it was, 'Pack your bags, gang, we're going home!' and no-one knew if we were ever coming back."
Julian Cress, The Block's creator and executive producer, admits shutting down the site was a difficult call to make, but that there was little choice.
Julian tells TV WEEK he had been in contact with state and federal government agencies for weeks to learn more about what was then a largely unknown threat.
"But boy, did we become experts very quickly," he says. "Because one minute it was a storm cloud on the horizon, and the next it was pouring rain."
Realising COVID-19 was rapidly affecting Australia, Julian and The Block team began planning for a possible stop work, then pushed those plans into action when New Zealand, and then London, suddenly locked down from the rest of the world.
"We had been told that if Australia was going to shut down we'd get plenty of notice, but then that night NZ and the UK both initiated lockdowns with no notice," Julian says.
"I realised that could also happen here. The danger for The Block is that our contestants are from all over the country and, for the most part, are parents of small children. If a lockdown was introduced, they would be stuck here, thousands of kilometres from their children. I didn't want to have that on my conscience or on that of the show."
That led to Cam whistling together the Blockheads for his memorable "Pack your bags!" speech.
"Our decision was the only one that could have been made," Julian says. "And we really didn't know what was going to happen next!"