Complex sentence: At RNZAF Station Wigram in the outskirts of Christchurch was Training Group RNZAF (active by May 1948 to the early 1970s at least) responsible for training and support.
Split into simpler sentences: At RNZAF Station Wigram in the outskirts of Christchurch was Training Group RNZAF, active by May 1948. It was operational to the early 1970s at least, responsible for training and support.