Signatories agreed to share intelligence and strengthen law enforcement cooperation, among a host of other joint initiatives.
Australia and Asean have inked a deal for lending a hand in tackling terrorism and extremism funding.
Richardson urged Prime Minister Malcolm Turnbull to take a stronger stand against excessive Chinese territorial claims by sailing within 12 nautical miles of Beijing's man-made islands in the disputed waters.
President of the Cambodian Australian Federation and Member for the Melbourne seat of Clarinda, Hong Lim said Hun Sen's visit is "just not acceptable".
At a private Asean leaders' dinner right after the conclusion of the conference earlier today, Turnbill positioned himself next to Najib, where the two leaders shared their insights on issues of mutual concern.
Najib said the special summit, which is the first in its series, indicated that as an organisation and as a region, Asean was getting more and more recognition from non-member countries.
"There are also places and restaurants owned by Malaysians here (in Australia) and this proved the close people-to-people ties between Malaysia and Australia", he said.
Britain launches murder probe into death of Russian exile Nikolai Glushkov
The Russian embassy has said it sought Glushkov's extradition in 2015, but the British government refused, Sky News reported. The anti-Putin Russian exile who was found dead in London earlier this week was murdered, British police have confirmed.
This is the first time Australia is hosting a summit with all ASEAN leaders.
In his speech, Turnbull told the global audience tuned in on the plan the region had drafted in the war against terror that "Najib was among Australia's best allies in the war against the scourge". "Being a diverse society is attractive and it will be wonderful if we all respect each other", he said.
Malaysia, he said, used the National Security Council, Prevention of Terrorism and the Security Offences (Special Measures) Acts, as it did not want to wait for atrocities to be committed before taking action.
The MoU signed between the countries entails cooperation to take place in putting together and launching counter-terrorism legislation.
The $56 million summit has turned Sydney's Darling Harbour conference centre into a fortress as security forces patrol the area to ensure the event goes smoothly for about 4,000 government officials, business leaders and other attendees.
Foreign Minister Datuk Seri Anifah Aman and his counterpart Julie Bishop sat in with the leaders.
They also had a New Colombo Plan afternoon tea session at which the ASEAN heads of state and government met Australian students who have pursued higher studies under the plan.