What happens if land measurement comes short

Hi all,
A hypothetical sequence of events that can happen

1. Surveyor surveys an estate lot to say 400 sq.m and puts pegs as measurement
2. A buyer puts an offer on that land thinking it will be 400 sq.m and puts a deposit.
3. Land titles, bank values it, loan settles, land deal done.

4. Builder moves in, puts slab and commences construction.
5. Halfway through they determine something is not matching up.
6*. They call another surveyor who measures it again and it comes short to say only 385 sq.m.
7. Neighbouring houses are already built, its too late to move boundaries.

(*Somewhere those pegs got moved by someone)

Buyer is gutted and feels duped.

Is there a step which is missing in this whole sequence, which ensures that this shouldn't happen.
If yes, who does it and when?

Cheers

Isn't the land ownership determined by what's on the plan/title, rather than where the pegs are? So the buyer still owns the land, and the neighbors have knowingly or unknowingly built on the buyer's land.

Bingo!
strannik is right. You own the land on your title. If the title says 400m² then that's what you own regardless of what or how the neighbours houses are built.

Every Builder I have known won't start until all the boundary marker pegs have been located and checked.

If the pegs are visible its a ten minute job to check the dimensions to confirm nothing has been moved.

For my last build the builder told me that the pegs were missing and I would have to pay for a resurvey. . . . but armed with a 30m tape measure and spade I located and checked them very quickly. (https://anewhouse.com.au/2020/08/block-boundary-pegs/)