The referendum results announced on Friday do not include an estimated 480,000 special votes, including overseas ballots, so the final outcome will not be confirmed until 6 November. But with such strong support, the decision is not expected to change.
The referendum on assisted dying was held alongside the general election earlier this month. In a separate non-binding referendum held at the same time, New Zealanders narrowly rejected a proposal to legalise recreational cannabis.
The preliminary results on the cannabis vote was 53.1% no and 46.1% yes - though this result may be subject to change when the special votes are counted.
The "yes" verdict had been anticipated after polls suggested strong public support for the law, which was also backed by Prime Minister Jacinda Ardern and the opposition leader, Judith Collins.
Ms Seales was a lawyer who launched a legal challenge for the right to end her life with medical assistance after she was diagnosed with a brain tumour. But her case was unsuccessful and she died of her illness five years ago, aged 42.