A common work arrangement these days is for a worker to be employed by a temporary employment agency – often also called a “labour-hire” agency – and to be hired out short-term to other employers.
It’s called “triangular” employment because there are three parts to the arrangement:
you the worker, who’s employed by
the labour-hire agency, who hires you out to
another company (the “host business”), such as a construction company.
Your employment contract is with the labour-hire agency, not the construction company, so the agency is legally your “employer”. But even so, while you’re hired out you’ll be working under the immediate supervision of the other business, the agency’s client.
If something goes wrong while you’re working for the other firm, the law says that you can take a personal grievance against them even though technically they’re not your employer (because the agency is your employer). For more information on triangular employment, see the chapter “Resolving employment problems”.
Who’s who: “Controlling third parties”
The law calls the host business a “controlling third party” because they’re the third part of the triangle along with you and the agency who employs you, and because they in fact control your day-to-day work, even though they’re not legally your employer.