The competition, marking the garden's second birthday, has the theme "our own oasis in the city".
Manager Margo Winchester said participants could enter the garden free of charge until the competition ended next Friday.
The garden opened in July 2008.
Dunedin City Council community life general manager Graeme Hall said last November visitor numbers averaged about 8000 a month during the first year.
However, numbers had dropped since, he said, with 3600 people visiting in October last year and about 2600 in each of July, August and September.
Ms Winchester said 1109 people visited last month.
She did not have complete visitor numbers for the 2009-10 year.
"We had a good first year of operation and that was a bit of a honeymoon period. But last year wasn't quite as good, so we're working on boosting visitor numbers. This competition is certainly part of that strategy."
All entries would be displayed in the Wall Street mall the week after the competition finished.
"Images of the garden are a fabulous way of getting people interested in what is here and we're working on getting a variety of pictures of the garden."
The winning photograph might be used in publicity material, she said.
- Annabel Reid
