The colony is located under the bridge where Dundas St. crosses the Credit River. About 25 tubes go into the concrete wall (I assume for drainage) providing extremely secure nest locations. There are several adequate banks for Bank Swallows within 100 m of this bridge but as far as I've been able to tell they are unused.
At least one of the holes is occupied by Northern-rough-winged Swallows and another by House Sparrows, but the rest are available for Bank Swallows.
Northern Rough-winged Swallow
I only saw Bank Swallow using 5 or 6 of the holes in the half hour I watched the colony, but given the number of Bank Swallows using Erindale Park I suspect at least 10 are occupied.
I have not been able to find any reference to Bank Swallows using similar nest sites in North America. Although this species frequently uses artificial locations for nesting (e.g. gravel pits), the nest tunnels themselves are usually dug into the substrate. The only report I've found of Bank Swallows using premade tunnels is from Breeding birds of Ontario: nidiology and distribution by G. Peck and R. James as cited in Birds of North America Online. 3 out 713 nest records (from the Ontario Nest Record Scheme) were from "plastic tubes in banks". Unfortunately, I have no access to the original publication right now so I have no more details.
There are however a variety of reports of this species nesting in a wide variety of artificial burrows in Europe, where it is called Sand Martin.
Clearly this is not a common behaviour this side of the pond, but I'm not sure exactly how unique it is. I may have found something fairly significant, or this may be a regular occurrence that has not been adequately documented in any source I've been able to find. Regardless, I was happy to confirm nesting at this location. All five species of smaller swallow (excluding Purple Martin) currently nest here, which is great to see for this rapidly declining group of birds.