The NEW values (or NEW_BUFFER as you have renamed them) are only available when INSERTING and UPDATING. For DELETING you would need to use OLD (OLD_BUFFER). So your trigger would become:
CREATE or REPLACE TRIGGER test001
AFTER INSERT OR DELETE OR UPDATE ON tabletest001
REFERENCING OLD AS old_buffer NEW AS new_buffer
FOR EACH ROW WHEN (new_buffer.field1 = 'HBP00' OR old_buffer.field1 = 'HBP00')
You may need to add logic inside the trigger to cater for code that updates field1 from 'HBP000' to something else.