Is it possible to replace just the cistern instead of the entire toilet set? I have had water leaking problem in my toilet. The water is running from cistern to the bowl whenever the cistern is fille

You have to check inside the cistern. Various valves may be contributing to leaking and constant running.

See this article:
https://www.familyhandyman.com/plumbing/toilet-repair/parts-of-a-toilet/view-all/

Are you saying that water is running into the toilet bowl? If so, check the rubber flapper at the bottom, and the valve (tube standing up).

The float valve needs adjusted or replaced.

Fixing a leaky toilet is actually rather simple, requiring few tools and just a little time and effort. In looking at Caroma toilets on Google, they appear to be just a standard type of unit, and should present few if any problems for yourself or any handyman of modest skill.

If it is leaking around the base of the toilet - where the toilet sits on the floor, the repair item is a simple wax seal. Here is a tutorial to effect this repair: Replace Toilet Wax Ring. The wax ring is very inexpensive, usually $5 to $10 USD.

If the water is leaking from between the tank (cistern) and the toilet bowl, the job is even easier, as it does not require lifting and moving the entire toilet. A good tutorial for that repair is here: How to Replace Tank to Bowl Gasket . The gaskets, new nuts, bolts and rubber seals come in a kit and also cost only $5 to $10 USD.

I am sorry to hear your Plumber insisted you buy an entire new toilet, but do remember he is in the business of making money!

sounds as though there may be a trickle of water going in to the pan from the cistern. To check you could turn OFF the water to the cistern, or tie the float arm up. then flsu the water in the cistern so that no more water will trickle in to the loo. Leave it a goodly time, several hours say, and monitor if it still runs for a few mins every hour. Actually running for two/three mins is rather a long time but first i would think you need to eliminate the possibility of the above steady trickle from the cistern.

check the donut seal which seals the cistern to the pan, they perish and leak when the toilet is flushed, water runs down back of pan to floor you can check this by drying/wiping with tissue back of pan and flushing you will see if any moisture is running down there by wiping again with tissue and seeing if it is damp.

I am not familiar with the model of toilet that you have, but many toilets suffer the same problem. It is usually due to the toilet cistern over-filling, as the water running into the bowl is the safety-overflow mechanism. The overfilling can be for a few reasons. The most common is that the fill valve is being held open, either due to mal-adjustment, or due to it being postioned slightly wrongly so that it hits either the side of the cistern or another part of the mechanism and can't close off properly. If you lift off the top of the cistern and watch what happens when the flush goes, you might be able to spot what is preventing the valve from shutting off entirely, and solve it. Hope this helps! :)

most modern toilets do not have an overflow pipe to outside instead they overflow into the bowl itself so have to adjust level so it does not overflow into return

The shut off valve depends upon where your plumber fitted it and whether one was installed at all. If there's not one near the cistern then you'll need to find the stop valve which is on either the main household cold water tank outlet or if the cistern is filled directly by mains water then you'll need to turn off the main supply valve or even the valve in the street.

The fault is caused by a failed syphon assembly inside the cistern. Although it's possible to remove the faulty part and replace it the sheer number of different syphon assembly designs and the age of many of them usually mean that it's quicker and simpler to replace the entire assembly. Without knowing exactly which make and model of syphon you have it's impossible to give you any more specific details as the operating methods vary so widely, but if you're mechanically minded then you may well find that you can dismantle the old syphon and locate the failed seal. Sometimes all you need to repair them is an old car tyre inner tube and a pair of scissors. A repair often costs pennies and takes hours, a complete replacement syphon assembly often costs between £13 and £60 depending on the model and design (plus labour) but usually takes no more than around fifteen to twenty minutes to replace if the cistern is close-coupled to the toilet and a bit longer if it's an old-fashioned high level cistern with restricted access.

Given the need for guesswork based on the limited details in your question I hope that you appreciate my reply and ask only that you return the favour by rating my answer.