Malcolm Turnbull had barely got through his predictable declaration that  Australia needed a clear economic plan to secure its future; the high-powered audience was happily digging in to the remains of its luncheon dessert, a chocolate confection with icecream … when the whole enterprise turned to custard.

At the tables reserved by the Committee for Economic Development (CEDA) for the media, chairs were tossed back and earnest young people leapt to their feet, shouting.

With dessert spoon poised in mid-air and icecream melting, I suddenly understood why I had not recognised the journalists at our table, or those adjoining.

Few of those wearing media badges were actually journalists.

Despite their business suits, quiet demeanour and carefully combed hair (which, combined, should have been a dead giveaway), they were all undercover rebels, and their rebel-yell was already deafening.

"Malcolm Turnbull, shame on you! Shut down Manus and Nauru," they cried, running hither and thither, dodging security guards, thrusting arms in the air and causing general startlement to Melbourne's gathered money-shakers.

The Prime Minister looked momentarily thunderstruck before being struck actually mute. Someone had turned off his microphone.

A woman brandishing a large placard clambered on stage right next to Mr Turnbull and began shouting words that might once have made a sailor blush. "Fer f....'s sake," she hollered. "Close the bloody camps."

In case you were deaf, her placard reinforced the message: "FFS, close the bloody camps", it echoed.

For what seemed long minutes, the Prime Minister stood staring at this apparition, apparently barely able to comprehend what was happening. Eventually, a protective agent reached the woman but seemed unable to decide whether to tackle her or reason with her. Eventually, he and others took her arm and marched her offstage.

How she had infiltrated this high-powered crowd armed with a placard - indeed, another woman near the stage waved an identical poster - seemed inexplicable. How others had strolled in with home-made media badges also had no immediate explanation.

This was the Prime Minister's first major economic speech since the election. CEDA had hired the splendid ballroom at the Grand Hyatt at the top end of Collins Street. The moneyed and the large-L Liberals had gathered.

Yet security had been breached. Spectacularly.

Order was restored, but all the wind had gone out of Mr Turnbull's economic manifesto.

Outside, there was still much caterwauling as security tried to clear the building. One of the women demonstrators sat upon the carpet, wailing and complaining, with the loud help of a colleague, of being assaulted. More language that might once have caused a wharfie to go pale echoed around the expensive foyer. The alleged assaultee climbed to her feet and stalked off.

The protesters later identified themselves as belonging to WACA - the Whistleblowers Activists and Citizens Alliance - who want an end to the detention of asylum seekers on Nauru and Manus Island.

Mr Turnbull was not thrilled.

The Prime Minister's Office has demanded a "please explain" from CEDA and Mr Turnbull's office also ordered the Australian Federal Police to provide a full "incident report" by Thursday morning.