Prof Mann, who recently stepped down as chairman of a Ministry of Health diabetes advisory group in frustration at the lack of action on the disease, said he was "substantially disillusioned" no National MP attended World Diabetes Day at Forsyth Barr Stadium yesterday.
He showed the forum "scary" University of Otago research, soon to be published, showing disease rates, which for the first time included pre-diabetes.
In the 55-64 age group, the rate of all forms of the disease (diagnosed, undiagnosed and pre-diabetic) was nearly 50%.
"If that isn't scary, I don't know what is scary," he said.
Public health programmes were needed to prevent pre-diabetics developing the disease, which was not where the Government had decided to focus, he said.
Prince Charles would have understood had National MPs not attended activities around the Royal visit to free up an MP to attend the forum, which was the only one of its kind in New Zealand, he said.
Prof Mann also took a swipe at the Southern District Health Board for serving unhealthy food at an unrelated workshop that happened to be running at the same time.
People should write to the board complaining about the food it was serving, he said.
This was brought to the forum's attention by type 2 diabetic Errol Sharp, who said he initially went to the wrong floor, and saw tables "groaning" with raspberry slice for morning tea.
This was an "absolute disgrace", Prof Mann said, as the unavailability of healthy food was partly responsible for the diabetes epidemic.
Dunedin National list MP and chief whip Michael Woodhouse said Parliament was sitting, and requests for leave had to be carefully considered to ensure enough MPs were present to complete the day's business.
Prof Mann, who he suspected did not understand the workings of government, needed to "calm down", Mr Woodhouse said.
"Would we say to Prince Charles: 'I'm sorry, these people won't be coming because somebody else has to go to a diabetes conference?"'
As whip, he granted leave for two MPs to attend Prince Charles-related activities.
No New Zealand government had done as much for diabetes as the present one, he said.
The only politician to attend the Dunedin forum was Labour MP and health spokeswoman Maryan Street.
She told the Otago Daily Times the lack of representation from National was an "appalling no-show".
The Government "dismantled" Labour's healthy eating and exercise reforms, including ones that cost nothing.
Diabetes New Zealand president Chris Baty said New Zealand was seeing "astounding" increases in diagnosed diabetes of up to 10% annually. The Government must show leadership, diabetics must be more vocal and the community at large needed to be educated, she said.