Air carrier Relief Pilot reported an altitude deviation on initial approach which resulted in a terrain warning from ATC. The crew corrected the descent profile and landed safely.

Narrative

I was the RO on a heavy crew. For the departure from ZZZ and arrival into SZX I was seated in the second observers seat behind the CA.After reporting for duty I noticed that there had been a crew change and an ACP (Assistant Chief Pilot) was listed as FO2. After he arrived; he said he would be the CA and the original CA would be FO2. When they initialized the ACARS; the CA change had not been made. I did not know that as I was conducting my preflight as RO. I noticed that he signed the company electronic document with the original CAs and I stated that the FOM said the assigned CA had to occupy his seat in the terminal area. They made some phone calls and got it switched. Additionally; the ACP had been listening in on a conference call during the brief and preflight almost up until door closure. We blocked out on time.Departure and cruise were uneventful.Once talking to approach we asked which arrival to expect several times. Finally; after passing POU; we were assigned the SAR9ZA and RWY33. Around SAREX we were vectored off of the arrival for traffic and given a descent below the path; so the PF was using a combination of FLCH and VS to descend. We were eventually given direct to NLG; which was an approximate SE heading to rejoin. NLG was also a feeder for the ILS Y 33. We were given many step down descents to 2300 (700m) and began configuring towards flaps 5 to comply with speeds. On the base leg; after D8.8 SHK; we were cleared for the approach. The PF armed LOC to intercept and set the FAF altitude of 1800 in the MCP (Mode Control Panel). I believe we were in VS at the time; but I could not see many of the instruments from my seat. I noticed that we were starting to descend and mentioned that there was a WARNING on the approach plate that said Before intercepting LOC; Aircraft should keep 2300 or above. Immediately after saying that; ATC directed us to climb multiple times in rapid succession. After climbing back to 2300; LOC captured and we turned inbound. During the (left) turn we broke out of the bottom of the clouds and could see the buildings that were the reason for the warning. There was a mention of being above GS; but I could not see the ISFD from my seat so I was just following along on my approach plate and what I could make out of the FOs flight displays. Once established inbound we finished configuring and continued to land. During crew debrief we discussed what happened and what we could have done differently. The CA and FO2 said they saw a false glideslope initially. I could not see the ISFD and the FO said he did not see it. We also discussed use of different vertical modes. The CA said he would make sure that warning was added to the airport company pages to alert other crews. I asked if any reports were required and was told that it was not on the list of reportable events. The next morning; the CA reached out to us and requested that we send him a synopsis just in case. Before I could send it to him I received another email from an ZZZ ACP stating to send him a synopsis. I sent my summary to both the CA and the other ACP. They said that was all they needed but I still felt an official report was required so I reached out for guidance and they recommended that I fill out a report.