If my salary income is from overseas as I'm working overseas, does the eligibility for BTO still apply?

During the BTO application process, the Housing Development Board (HDB) will take into account your total household income, including your overseas salary. They will convert your overseas income to Singapore Dollars (SGD) for the purposes of the income assessment.
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If your total household income, after converting your overseas salary to SGD, exceeds the income ceiling for BTO eligibility at the time of your application, then you would not be eligible for a BTO flat. The income ceiling is periodically reviewed and adjusted by the HDB!

I've checked with HDB before, yes they do take into account pre-tax salary after conversion. However, since they probably do not have the information, it will be on a self submission basis so it will be at your own risk if they find out you were not eligible and decides to take back your bto in future.