Mugabe and an entourage of more than 30 arrived in Port Moresby amid tight security on Sunday night, and was met by PNG government ministers and officials.
There are 79 country members of the group, about 50 of which will be attending the event in Port Moresby.
Since 1997, heads of state and government of ACP member countries have met regularly to lay down the broad guidelines for the general policy of the group and give the Council of Ministers instructions relating for its implementation.
The summit is presided over by the head of state or government of the host country.
The first summit was held in Libreville, Gabon, in 1997 and subsequent events have taken place in Santo Domingo (Dominican Republic), Nadi (Fiji), Maputo (Mozambique), Khartoum (Sudan), Accra (Ghana) and Brisbane (Australia).
This year's summit will discuss the future of the ACP croup as a revitalised cohesive force advocating the interests of its member states in the international arena.
Discussions will take into consideration recent key international developments, including Agenda 2030 and the Sustainable Development Goals, issues of migration, climate change and the fight against terrorism.
"This is a very important event, taking place at an important time in the history of the ACP Group. The future of the organisation, in terms of where it goes after the expiration of the Cotonou Agreement [the comprehensive agreement between the ACP Group and its main development partner, the European Union] in 2020 is a fundamental issue with flow-on consequences … it's about legacy creation," said Foreign Affairs Minister Rimbink Pato.
"Papua New Guinea regards it as a privilege and an honour to welcome representatives from all the ACP states, so that we can address together the challenges that we face as individual members, as a group, and globally, especially in terms of the 2030 Agenda for Sustainable Development," Pato said.
Under the theme ‘repositioning the ACP to address the challenges of sustainable development', leaders will focus on three main areas: equitable and sustainable development for ACP peoples; enhancing the role of the ACP in global governance for development; and peace, security and political stability as a prerequisite for development.
The summit will also receive and deliberate on the final report of the eminent persons group, chaired by former Nigerian President Olusegun Obasanjo, on the future of the ACP group. Since its formation in 2013, the eminent persons group has held consultations in all six ACP regions with various strata of society, and conducted in-depth research, culminating in a number of recommendations for reform, in order to reorient the ACP to deliver more effectively on the development ambitions of its member states.
The 8th Summit coincides with ongoing discussions on the future relationship between the ACP Group of States and the European Union. The decades-long partnership is currently outlined in the ACP-EU Cotonou partnership agreement, which is due to end in 2020. Formal negotiations for a follow up framework are expected to commence in 2017/2018.
"There is a real opportunity here to make history, in contributing to the global development agenda by enhancing the voice of developing and marginalised countries on the international stage. The ACP group seeks to enhance its capacity as a hub for south-south and Triangular cooperation, leveraging its large membership and its long experience in international cooperation amongst member states as well as with the European Union," said ACP secretary general Patrick Gomes.
Aside from national leaders, heads of international organisations are also expected to attend. The summit's plenary session yesterday and today, preceded by a meeting of Ministers of Foreign Affairs on Monday.