It is not really a problem for the SPP if the share price falls below 1.80 as the issue price will adjust if necessary.
"The offer price of these Shares will be the lower of the Placement offer price of NZ$1.80 and a 2.5% discount to the five-day volume weighted average price of Heartland shares traded on the NZX over the five business day period prior to, and including, the closing date of the SPP offer"