A teenager has spoken of her devastation at losing her 18-year-old boyfriend to cancer, just four weeks after a diagnosis that gave him up to six months to live.

Zoe Meadowcroft's partner Lucas Issa had been battling various forms of cancer while he attended high school in Lewisham, in Sydney's inner west.

But on Friday - less than a month short of his 19th birthday - he died in hospital surrounded by his family and distraught girlfriend.

'I didn't believe we would only have days left together when we were supposed to have months. We were supposed to grow old together,' Ms Meadowcroft said.

She said she has 'prayed for a miracle' every day since his diagnosis.

Lucas had been given between three and six months to live only four weeks prior to his death, after a trip to the emergency room found a tumour on his spine and another eight scattered throughout his body.

She said despite the diagnosis, she never thought Lucas would 'lose his battle', and has since created a GoFundMe page to help raise funds for his family.

'Lucas fought the toughest and longest battle with cancer, and never did I think I'd be here writing this today, none of us did,' she said.

'We were so happy together, everyone that knows us knows there wasn't a minute we were apart.

'We were so in love and nothing was ever going to take that away from us.'