Nelson police says they're still trying to piece together what happened in the lead up to the death of a 15-month old Greymouth boy.

Leith Hutchison died almost three weeks ago after he was taken to Grey Base hospital in cardiac arrest.

Medical staff later found the boy was suffering from a bleed on his brain.

Detective Senior Sergeant Dan Keno says the results of a post mortem examination aren't expected for several weeks.

Investigators have spoken to family members and anyone who had been in contact with the boy in the last few weeks of his life.