Social media platforms, such as Twitter, offer an accessible way for people to share information and perspectives on a wide range of topics. Such citizen discourse can be a valuable source of information and offer policymakers and researchers insights into public sentiment, needs, and suggestions, guiding more informed and responsive planning and policy decisions. In this paper, we propose a novel approach using Large Language Models (LLMs) for data augmentation and multi-class classification to extract domain-specific data from tweets and identify issues raised by citizens thus providing policymakers and social science researchers with valuable data to formulate effective plans and policies for improving services. This approach involves initially collecting data from Twitter using specific keywords and manually labelling a subset of the acquired data. Then, we introduce a new data augmentation strategy employing a LLM that leverages the initial human-labelled data to enhance text diversity and address imbalances in the dataset. Finally, we use the manual-labelled and augmented data to fine-tune different LLMs to classify texts across multiple topics. We test our approach considering the identification of issues related to the cycling domain as case study, detecting tweets across eleven categories associated with infrastructure, safety, and accidents. Through fine-tuning BERT-based models and experimenting with zero- and few-shot prompts with GPT for tweet classification, we accomplished an accuracy of up to 90.9%.