I think you are referring to "turnkey finance" and expression not widely used in Australia. Rolf or Victor may correct me there.
Turnkey is an expression used to describe an entire solution, plant or piece of equipment. In real estate terms it would mean finance for the whole development (or house/land package) from acquisition through construction to completion.
The buyer/borrower does not have to do anything at the end -- just turn the key and go.
If a financier or sales agent used the expression "key turn financing" run away.
1) They don't know what they're talking about.
2)They are using professional sounding expressions to make you feel stupid, hoping to build your faith in them as experts. Telemarketing stock scams from overseas use the same technique (sound more knowledgeable than the punter).
I don't buy house and land packages so this is hearsay.
I've seen them structured both as a lump sum payment at the end and a pay at milestones. The builder has legitimate expenses during construction, so paying at the end will be more expensive because the builder carries the cost and risk.