"The police agreed it would have happened whether we were there or not. It was two drunk people. We were just witnesses to it rather than anything else," Ms Manning said.
"Neither of them had had anything to smoke as far as I know. They hadn't been with our group. I'd say if they had had a smoke they probably wouldn't have ended up doing it," Ms Manning said.
She had hired security guards for the past four years' J Day events but had decided against doing so this year because the previous rallies had been trouble-free.
The event itself went very well, Ms Manning said, attracting a lot of attention for cannabis law reform. It involved "lighting up in protest" but despite cannabis being smoked openly no arrests were made.