This could be susceptible to a rainbow table attack, especially if you can collect lots sequential ones.
For example, if I have a rainbow table of hash(0), hash(1000000), hash(2000000).... which would be 1/1,000,000th of the size of the full rainbow table, then I could look up each value coming out of your sequence in my rainbow table. Once I know one, I know all previous and all future numbers. It would take, on average, 500,000 numbers from your sequence that matches.
Similarly, if any number you hashed coincedentally occurred in a rainbow table, then all would be lost (future and past). For example, 555555555 is in crackstation, so if you went past that, you'd be sunk.
I tried running some hashes of random numbers through crackstation - 7 digit random numbers seem to be precalculated, 9 digits not.
If you started at a random 64-bit number (about 19 digits), then you might get away with it, almost all of the time.