If travelling from overseas, you can fly from Australia direct to Queenstown or Dunedin – both airports are about an hour by road from each end of the track, Clyde or Middlemarch. Christchurch is the main international airport in the South Island.
You can hire rental vehicles at each airport, or catch shuttle buses to/from either end of the Trail. The closest airport to Clyde is Queenstown – it is called “the Adventure Capital of the World” and would be an exciting add-on to your trip.
If starting from Dunedin airport you could catch a shuttle into the city (Dunedin is a beautiful Victorian city, its Otago Peninsula is popular with eco-tourists), then travel to the Middlemarch end of the Trail on the scenic Taieri Gorge Railway.
Energetic cyclists can consider cycling 14 km from the airport to catch the Taieri Gorge train at the Wingatui Station in Mosgiel – but check the route and you will need to pre-arrange a pick-up.
You could also plan an airport-cycle-airport trip – fly into one airport and fly out from the other.