From the Hillman website:

Sunday March 15  –  The Historic Motor Vehicle Gathering at Park 22 in the park lands

Organised by The Vintage Sports Car Club of South Australia, the annual Historic Motor Vehicle Gathering is held on the 3rd Sunday in March every year.  Location: netball courts in Park 22 between Anzac Highway & Goodwood Road in the park lands, from 10 am to 2 pm.  Free entry.
Gather at 09:45 am on the southern side of South Terrace, opposite Vinrace Street, preparing to depart as a convoy to the car park at 10 am.
Assembly map:  [Google Maps with Street View]
Car park map for late arrivers:  [Google Maps with Street View]
Entrances are on Anzac Highway (south of the pedestrian traffic lights, approach only from the CBD and Goodwood Road (approach only from Greenhill Road).