Rob and Cassie met and married in the USA in 2000. Rob is American and grew up in New York but agreed to come live in Melbourne where Cassie's family live and to allow her to finish her degree in English literature.

They subsequently had two children, Isabelle aged 16 ('Izzy') and Elizabeth aged 13 ('Lizzy').

After 18 years of marriage, Rob and Cassie decided that their marriage lacked the happiness that it brought in the early years, and both felt that they would be happier with other people. Their separation was relatively amicable. They were able to come to an agreement whereby the girls would live equally between them. Both parents lived relatively close to the girls' school and the girls were of an age where they could travel by tram to and from school to their parents' homes.

In February 2018 they filed Consent Orders with the Family Court in relation to the parenting arrangements.

During the 2018 summer holidays, Rob took Izzy and Lizzy to New York for a holiday. Cassie had agreed to the holiday and thought it would be a good experience for the girls. However, once there, Rob realised how much he missed New York, the culture and his family. He decided he wanted to stay and asked the girls if they would like to live with him. Izzy and Lizzy were thrilled and said yes immediately.

Rob sent Cassie an email to let her know about the new plan. This was followed by several FB Messenger texts from each of the girls, with Rob's encouragement, saying how excited they were about living in New York. The messages included photographs of them at the various tourist spots in New York.

Cassie is devastated. She immediately contacts the AG's office to seek assistance to commence Hague Conventionproceedings. However, she was advised that the process was lengthy and that she should consider also filing an application in the Family Court of Australia.

Cassie does so immediately, and the matter is listed for an urgent interim hearing before Justice Joan's.

What are the considerations that must Justice Joan's must take into account when determining whether to order Rob and the children to return to Australia? Please relate it to the Family Law Act 1975