Burying the dead in separate coffins in the same pit
Burials in Australia involve putting the deceased in a casket and placing it in a 6 or 7 m deep pit. Subsequent interments are made atop the previous caskets which are then separated by a layer of earth.
The question is: Is this method of burial compatible with Islamic law? We would like to know the ruling so that we could notify [authorities] of the burial method that complies with Islamic law. Please note that there is no need for interring the dead in the same plot as there is no shortage of land.
In Islamic law, the body of a deceased is interred in a pit to conceal its odor and protect it from any abuse or tampering. We opine that the manner of burial described [in the question] does not involve any legal proscription but is compatible with Islamic law.
Though casket burials are deemed disliked in Islamic law, this dislike is waived when there is a necessity. In Hanafi jurisprudence, it is categorically permissible to bury a deceased female in a casket, whether for a necessity or otherwise. Muslims living in non-Muslim countries need to assimilate and co-exist with the societies in which they live without violating the customs of these lands provided they do not contradict the rulings of Islamic law. There is no objection in Islamic law to any method of burial followed in a particular country provided it does not contravene a definitive command. Islamic law does not seek to violate customs provided they do not conflict with a ruling over which there is a scholarly consensus. Imam Al-Sarkhasy, the Hanafi scholar, said, “Imam Abu Bakr Mohammed Ibn Al-Fadl (may God Have mercy on him) said, ‘There is no objection to this in our land because the earth is soft. He deemed it permissible to use wooden boards or caskets to the extent that it was said, ‘There would be no harm in using iron caskets in these lands.”
The basic principle is that each body must be buried in a separate grave and it is not permissible to bury more than one body in the same grave except for a need. A group of jurists have deemed this to be merely recommended and not obligatory based on its preponderance by some Hanbali imams. It is likewise the opinion of imam Al-Rafi’i from the Shafi’i school of jurisprudence. However, the scenario depicted in the question is different from that of burying more than one person in the same grave because the bodies are placed in caskets which are separated from one another by a barrier of earth, making this tantamount to burying the bodies in separate graves. Imam Al-Sarakhasi maintained that whenever it is necessary to bury two persons in the same grave, the bodies must be separated by a barrier of earth so that it is as if they are buried in two graves. Based on this, there is no harm in burying a male and a female in the same grave. There are however some scholars who deem this to be permissible even in the absence of a necessity. Proponents of this opinion include Imam Ahmed and Abu Mohammed Ibn Hazm. The erudite scholar, Al-Mardawi said, “He [Imam Ahmed] said, ‘It is disliked.’ This opinion was chosen by Ibn ‘Aqeel, Sheikh Taqiy Al-Deen, and others. He [Ibn Muflih] said in Al-Furu’: ‘This is the preponderant opinion which Al-Majd deemed to be the sounder opinion … He [Al-Majd] did not mention a conflicting opinion, indicating that he determined that imam Ahmed had only one opinion [on this matter] which: it is not prohibited’ End of quote. He [Imam Ahmed] said that it is permissible. This was reported by Abu Taleb and others. He [Imam Ahmed] said, ‘This is permissible with regards to non-marriageable kin.’ It was said that this is permissible with regard to minors of less than seven years; A-Majd declared in his Sharh that this opinion conforms with the principles of the school” End of quote. 
Imam Ibn Hazm wrote in Al-Muhalla (3/337), “It is permissible to bury two or three persons in the same grave and the one who is the most learned in the Qur`an is to be entered into the grave first.”
Based on the above and in reference to the question, there is no objection to burials conducted in the manner mentioned in the question. There is no need to ask Muslims to change the manner of such burials and contravene the customs of the country in which they reside provided the deceased are buried in Muslim cemeteries that are separate from others.
And God the Almighty knows best.
 Al-Mabsut (2/62).
 Sharh Al-Siyar Al-Kabeer (1/234).
 Al-Insaf (2/387).