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The greatest mistake to be made would be putting plans on hold with the idea of "dusting them off in five years" when things were better, he told a public session at the Dunedin Public Library this week.
The city could not afford to take its "strong CBD" for granted, he said.
"You've got a fabulous built resource and city structure that has been messed with and wounded in areas but is fixable."
Mr Mentz, of Urbanism Plus, was reporting back on the concepts a group of New Zealand and Australian planners had come up with after taking on board the views of Dunedin residents, businesses and the city council.
The aim was to come up with concepts that could be included in the Dunedin City Council's draft spatial plan "Dunedin 2050" and a more detailed action plan for the CBD for the long-term plan 2012-13 to 2022-23.
Mr Mentz said the future growth and potential for the city was to the south, especially the "warehouse district" near the harbour.
Work should start with investigating how to solve the impediment of the Crawford St state highway.
"It will give a better cross connection within the city. I have a high degree of optimism that we'll get somewhere. To get Crawford St back would be a fabulous resource."
Next, stronger links between the Octagon and an improved Queens Gardens area - a creative quarter to make the most of the historic buildings - could begin.
"They might be a 15 or 20-year resource but you do not want to screw it up now with the cheap option or by not supporting private ventures."
George St was "pretty cool" by a lot of standards but was not a "comfortable place" for families. However, it was a classic case of where small interventions would work, he said.
Micro-initiatives around George St and improvements to bus services and parking arrangements could come next.
When the rest of the system was working, then improvements to the Octagon could be looked at, he said.
Any plans for the Octagon should be the last project the city attempted as it was 88% "OK".
"It can be put in cold storage until you see where the community is in a number of years time."
Dunedin Mayor Dave Cull said he was "blown away" by the work that had been done and the findings were "extremely exciting".
The group's final report was due at council at the end of the month.