Which is better among petroleum engineering and computer engineering in future? I scored 85% in PCM in 12th.

A gap of 1 to 2 years after 12th can be considered normal. Although the students should be able to address it via the valid documents. The reasons like health, finance, skill development or family are accepted. 

And if the gap after the high school Diploma is beyond 2 years, strong documentation is required to justify the same. The documents like which shows productivity for which an SOP can serve great purposes. 

Yes, it should not be hard for you to get admission to the University of Western Australia as an Indian student with 50% in class 12th. However, please note that it does not put you in the top applicants either. The general admission percentage requirement for international students is 50 to 65%.

Look at the other admission requirements - 

• English proficiency tests with IELTS (bands 6.5) or TOEFL iBT (82+)
• High school Diploma from a recognised board like CBSE or ICSE etc. 
• Some course-specific subjects for Engineering programs 

Top universities require a minimum of 80-90% in your 12th-grade board exams.

• Average universities often accept students with 60-75%.
• For highly competitive courses like Medicine, cut off is usually higher. 
• Strong SAT/ACT scores can also compensate for a slightly lower board percentage.

Countries like Germany and Norway are excellent options. Because public universities there charge little to no tuition fees.  The cost of living and education is significantly lower compared to the USA or UK. You must factor in living expenses and language requirements.

Class 12 marks are important for fulfilling the admission criteria and tie breaking criteria (if any) during admissions. Admission to Central Universities are conducted through CUET scores, however, candidates must fulfil the minimum marks requirement for admissions.