How old is the house? Verify the leak occurring where first:
1. After meter but before tangki
Close the stopcock heading up to the tangki at the back is the house to verify if the leak is between the meter and the piping underneath leading to the kitchen, laundry and the stopcock. If the meter is still running, confirmed its leaking in that area.
2. After tangki
Ensure no one is using the bathrooms kitchen or laundry etc. Close all stopcock leading to each bath. I assume there are 3 as normally there's another down branch from the tangki to the first floor toilet. This need 2 persons communicating with a phone or intercom via two cordless phone like walkie talkie - one going from bathroom to bathroom and the other observing if the water meter is running when the stopcock is reopened. The meter observer will call / intercom / walkie talkie when the meter moves. No call, means that bathroom is not leaking. The one opening the stopcock will know which one is leaking.
What he's suggesting I think roughly is to lay and route the pipe through the front of the house into the roof under the eave to the tangki with a branch to downstairs back yard and kitchen perhaps using the same stopcock (flowing in reverse). The kitchen is where it terminates, hence the cold supply into the kitchen sink and laundry will be from this new pipe. The old pipe is left in disuse. Some do seal it with a nut. My guess is this pipe will be exposed? Otherwise it should cost more as there'll be hacking involved. Did he give you the job scope?
The material used on the newly laid pipe must withstand the heat outdoor otherwise it will burst somewhere. Normally what I've seen is to lay it along the common wall/fence between the house using a saddle clip to fasten it at regular intervals as it goes up into the roof. An old neighbor route it to a rocket water filter then up the roof this way.
See if there is a stopcock in the downstairs bathroom. If there is one and it's verified not leaking, this section is unnecessary.
Hello Helcyon27, many many thanks for your comprehensive suggestion! The house is possibly around 13 years old. It sits just above a slope and there are no other house behind our terrace house. I do have a tree which has grown as tall as the house and it is around 5 meters from the main pipe outside the house. Im not sure if the roots can cause some pipe to buckle and leak. Anyways i will do as you suggest first with the plumber. Much appreciated.