Complex sentence: The Victoria Park Tunnel is a motorway tunnel completed in 2012 on New Zealand State Highway 1 in Auckland, New Zealand, taking northbound traffic off the Victoria Park Viaduct, which was converted to all southbound traffic.
Split into simpler sentences: The Victoria Park Tunnel is a motorway tunnel built in 2012 New Zealand State Highway 1 in Auckland, New Zealand. When finished in 2012, it took the northbound traffic off the Victoria Park Viaduct, which was then converted to all southbound traffic.