So will there a once off increase in base salary, ahead of the 2019 changes?

I don't think there is any increase. It is merely splitting up the bonus into 12 months to pay you.

Last time: 5k monthly, total package 18 months = 90k per annum
New pay: 6k monthly, total package 15 months = 90k per annum

So still no 13 month right?

More like 13-14 months for avg performer. PB Quantum reduced since large part of grp bonus now form part of base pay.

I am wondering where people get the impression that 18 months package was the norm. Perhaps that is true at early 2000's or till early 2010's.

The last few years had always been about 15.5 months average, for an average performer.

With the salary package modified to removed the bonus at Dec which is usually 2 months+ abit. Yes, the reality is 1.75 months for average performer.

which means 12+1.75 = 13.75 months salary package is what you can expect for an average person.

So does MAS pay well for midcareers? If MAS offers an Associate position for someone 3,4 years of exp, how much should he expect?

that sucks because you receive lower Employer CPF contributions

Typically for Associate range is anything from 3k to 4.5k.

Type 1 - No Degree
If you have 3-4 years experiences w/o degree, likely 3-3.5k

Type 2 - With Degree
If you have 3-4 years experience w/degree, likely 4k and above.

Are MAS Officers considered civil servants and eligible for those year end AWS bonus declared recently? Understood from here that MAS has wide discretion and does not pay 13th month. So just wondering.

MAS is under SB (s://en.wikipedia.org/wiki/Statutory_boards_of_the_Singapore_Government)

All SB are considered Public Servants.

For Civil Servants, typically are directly working for govt. For example ministry of manpower (MOM), ministry of health (MOH), Mindef, etc.

s://en.wikipedia.org/wiki/Singapore_civil_service

Back to your question, MAS salary package is now basic + performance bonus. This is in line with Banking industry.