I've never heard of it before, from Australia.
Wikipedia, says "tiffin" is:
Tiffin, a light meal eaten during the day.
In the disambiguation page. In the article it says:
Tiffin is an Indian English word for a type of meal. It can refer to
the midday luncheon or, in some regions of the Indian subcontinent, a
between meal snack, or in South Indian usage, a light
In the introduction it does not mention it as a lunch-box as you say, however further down:
In Mumbai, a school-going child's lunch box is fondly called a Tiffin
In addition, the lunch boxes are themselves called tiffin carriers,
tiffin-boxes or just tiffins.
There is also an article for tiffin carrier. It looks like this:
From South Asia, they spread to and now are widely used in India,
Indonesia, Malaysia, Thailand and Singapore as well.
In dictionaries there are the following definitions:
A meal at midday, especially in South Asia.
(Cookery) (in India) a light meal, esp one taken at midday
verb (used without object)
2.to eat lunch. verb (used with object)
3.to provide lunch for; serve lunch to.
Wikipedia lists tiffin as also being the container for the tiffin, but none of the dictionaries I've listed do, they say it is the tiffin itself, (the food).
Wikipedia article on tiffin
Wikipedia article on tiffin box