WSRG will pay $805 million for the toll road.
The infrastructure business generates net proceeds of around $402.5 million for MIG.
The price will deliver for investors an internal rate of return of 25.9 per cent.
"MIG will continue to look to realise value for security holders by pursuing further opportunities to divest other road businesses," MIG chief executive John Hughes said.
WSRG is owned in equal parts by MIG and an Australian institutional investment manager.
MIG was holding on to its 50 per cent interest in WSRG pending ongoing discussions regarding a sell-down of that interest to third-party investors, Hughes said.
