剛剛係網上 submit 左個 application
但係 instruction 話要交 supporting document 去 hku
咁 supporting doc 係咪可以過左 31/1 (application deadline) 之後先交架 ?

可以過左1月31日先交supporting document, 我試過...
但係最好盡快交.

最好盡快交. or else it may not be taken into consideration even though its explicitly said

After you have submitted an online application, please follow to mail us all the
supporting documents which should include
- a copy of your passport and/ or HKID card
- all your academic transcripts (if you are currently studying in a tertiary
institution, please submit an official transcript)
- resume (optional)
- one reference letter/ one reference report using HKU standard form

the supporting documents are to show them that you didnt make up the grades.