The agreement was signed amid a week-long Australian visit by Solomon Islands PM Manasseh Sogavare.
In June, Australia ended a 14-year peacekeeping mission in the Soloman Islands following a civil conflict that brought it to the brink of collapse.
Mr Sogareve said he he hoped the new deal would never be acted upon.
"The treaty is about if we fall back into a situation where we were in the 2000s, Australia would come back and assist us," he was quoted as saying by The Guardian.
But he said "we will not allow the country to go down that way".
Australian PM Malcolm Turnbull said the deal would "enable defence, civilian and civilian personnel to deploy operationally in emergency situations to provide security or humanitarian assistance at the Solomon Islands government's request".
The new treaty comes two months after the conclusion of the Australian-led Regional Assistance Mission to Solomon Islands (Ramsi).
Ramsi had been a response to a Solomon Islands crisis - known locally as "the tensions" - which saw fighting between militants from Guadalcanal island and the island of Malaita between 1998 and 2003.
Australia and the Solomon Islands have signed a deal allowing Canberra to deploy security forces to its Pacific neighbour in the event of civil unrest.