Ms Fitzsimons spoke about ''Enough: the challenges of a post-growth economy'', in a 5.30pm public address to about 250 people at the University of Otago Castle 1 lecture theatre.
''There's a big change in our way of life coming. It's coming whether we like it or not.''
She urged people to pursue a more sustainable and egalitarian society before negative changes were forced by circumstances in the future.
Ms Fitzsimons, who was born in Dunedin and grew up in Mosgiel, said she always attracted her largest audiences when she gave public talks in Dunedin, and was not sure why.
She warned of mounting signs of global warming, and of extreme regional weather events, such as the recent hurricanes which had hit the United States.
And she also highlighted growing problems with the global oil supply.
Some of the early oil finds, such as in Saudi Arabia, had proved easy to access and extract, with the energy equivalent of one barrel of oil required to extract 100 barrels from the ground.
But recent less promising finds were now yielding only three to five barrels of oil for every barrel expended in energy to obtain it.
She warned against the ''thrashing of the tail of the dinosaur'' as the end of the previous unsustainable approach was accompanied by increasingly damaging attempts to exploit natural resources.
During a later question-and-answer session, she said she was ''appalled at the amount of hostility'' involved in repeated criticism of ''dole bludgers'' and said corporate tax evasion deserved more attention.
Human psychology and attitudes to consumption would prove crucial.
If people continued to fixate on accumulating material possessions they would ''keep consuming''.
Yesterday's talk was part of a series of lectures by Ms Fitzsimons on the same topic, organised throughout the country by the Quakers. Yesterday's event was co-hosted by the Otago University Centre for Sustainability.
