I am Master student from IIT and working as Researcher in TATA STEEL.
Which one is better? (1) Full time PhD at University of Melbourne, Australia (2) Part time PhD while working as Research Associate for Rolls Royce Project at Nanyang Technological University, Singapore.
I got the admission in University of Melbourne with Full Scholarship .I have also got selected as RA in Nanyang Technological University , Singapore
While working here i can also opt for part time PhD.
Here i am confused which one to join as my primary motive is to do PhD

Might be best to take the fully funded one. Part time PhDs can be very stressful.

There are a number of things you'll need to consider when choosing a program.

1) Geographic Location: Singapore and Australia, you'll need to decide which country you would prefer to live in. You haven't stated where you are located. Will you need to relocate? Fuss about with student visas?

2) Support Networks: In line with the geographic location, where are your support networks? Will either program require you to develop a whole new support system? A phd program, whether full or part time, can be an isolating and difficult experience, you may want to think about the kind of support networks you have in either location.

3) Experience vs Time Completion: Your fully funded would take 3.5 years to complete if you stick with the funding time allowed. A part time will take longer. HOWEVER, your part time is offering what appears to be VERY VALUABLE experience as a research associate that you may or may not get with the full-time program. You may or may not be offered RA work, or teaching work with the full-time.

While you have stated that your primary goal is to complete a PhD, you need to think about what it is you want to achieve beyond that. A PhD is not a guarantee for work experience or a job, and should only be considered a stepping stone to something else. Many students are struggling to find work because they are overqualified and under experienced, a PhD is not often considered work experience unless there was a work component.

What would I choose? As someone who is going to submit next week, has done a few job interviews to be told that I was a very close second because I don't have enough experience, I would actually take the Part-Time with the RA work because I would then have additional experience that would be helpful once I completed the degree.